What do you do with a warship?
-or maybe-
why are ships designed that way?
The vast majority of the time, military ships aren't being used as part of a huge fleet on fleet battle. Militaries need to do other things as well, and large multi-role ships will be designed with this in mind.
Freedom of Navigation Operations
This is probably going to be relevant mostly areas where a lot of different groups have space colonies. FNOPS are basically just sending a ship through an area that's supposed to be free for anyone to use to make sure that the area remains free for anyone to use. This would be especially useful if someone decides to declare themselves owner of Jupiter, and you need to go in there to make sure these people aren't harassing people flying near Jupiter.
Orbit Guard
As more ships start to move about in space, more debris are going to be knocked around star systems, and you're going to need constant monitoring of where all that stuff is. Military ships will likely also get the job of destroying or deflecting all that stuff to protect planets, moon-bases, space stations, even other slower ships that just might not be able to easily get out of the way or something.
This will likely mean most ships will be equipped with some small directed-energy weapons to nudge space-trash. Larger ships may carry a few nukes, tugs, or other specialized gear for larger asteroids.
Sustainment
Logistics. The larger ships are going to have a good amount of cargo-space. This would likely hold equipment for their own missions, as well as provide some capacity to do logistic operations like bringing important equipment to far off bases or emergency resupply of ships which brings us to.
Fuel Rat
Kinda in the same vein as the Orbit Guard, there will be situations where ships find themselves without enough fuel. Imagine if there is mechanical failure or damage which causes a leak in a freighter's fuel tanks? They might be running on slim margins and not have enough to reach the proper orbits. A military ship would likely be the one called on to intercept the damaged ship, repair the fuel tank, and provide it enough fuel to reach a safe orbit.
I would expect to see most military ships having fuel-transfer capabilities as well as some repair-tugs carried onboard
Customs Interdiction
Fast ships might constantly be trying to skirt the law and smuggle stuff between space stations or planets. Ships will constantly need to be intercepted and inspected.
This would require a lot of different equipment to do. The larger ship is going to need weapons, sensors, a complex CIC, marines, and a way to get those marines to a ship to investigate it. The ship may also not be fast enough to catch smaller fast shuttles going between space stations so they may also carry small, high acceleration, fighter craft with a short range alongside transports to move marines around.
Communications
As you get further away from built up areas, communication is going to become a problem. Large ships will naturally have long range communication systems but interplanetary or even interstellar systems are going to be far too big for smaller craft to carry. Larger ships are likely going to act as communication bridges for smaller craft, listening to their requests and relaying it around.
-or maybe-
why are ships designed that way?
The vast majority of the time, military ships aren't being used as part of a huge fleet on fleet battle. Militaries need to do other things as well, and large multi-role ships will be designed with this in mind.
Freedom of Navigation Operations
This is probably going to be relevant mostly areas where a lot of different groups have space colonies. FNOPS are basically just sending a ship through an area that's supposed to be free for anyone to use to make sure that the area remains free for anyone to use. This would be especially useful if someone decides to declare themselves owner of Jupiter, and you need to go in there to make sure these people aren't harassing people flying near Jupiter.
Orbit Guard
As more ships start to move about in space, more debris are going to be knocked around star systems, and you're going to need constant monitoring of where all that stuff is. Military ships will likely also get the job of destroying or deflecting all that stuff to protect planets, moon-bases, space stations, even other slower ships that just might not be able to easily get out of the way or something.
This will likely mean most ships will be equipped with some small directed-energy weapons to nudge space-trash. Larger ships may carry a few nukes, tugs, or other specialized gear for larger asteroids.
Sustainment
Logistics. The larger ships are going to have a good amount of cargo-space. This would likely hold equipment for their own missions, as well as provide some capacity to do logistic operations like bringing important equipment to far off bases or emergency resupply of ships which brings us to.
Fuel Rat
Kinda in the same vein as the Orbit Guard, there will be situations where ships find themselves without enough fuel. Imagine if there is mechanical failure or damage which causes a leak in a freighter's fuel tanks? They might be running on slim margins and not have enough to reach the proper orbits. A military ship would likely be the one called on to intercept the damaged ship, repair the fuel tank, and provide it enough fuel to reach a safe orbit.
I would expect to see most military ships having fuel-transfer capabilities as well as some repair-tugs carried onboard
Customs Interdiction
Fast ships might constantly be trying to skirt the law and smuggle stuff between space stations or planets. Ships will constantly need to be intercepted and inspected.
This would require a lot of different equipment to do. The larger ship is going to need weapons, sensors, a complex CIC, marines, and a way to get those marines to a ship to investigate it. The ship may also not be fast enough to catch smaller fast shuttles going between space stations so they may also carry small, high acceleration, fighter craft with a short range alongside transports to move marines around.
Communications
As you get further away from built up areas, communication is going to become a problem. Large ships will naturally have long range communication systems but interplanetary or even interstellar systems are going to be far too big for smaller craft to carry. Larger ships are likely going to act as communication bridges for smaller craft, listening to their requests and relaying it around.