I've long thought that the giant interstellar wars in most sci-fi don't make much sense in that they serve little economic and political purpose. Fighting over territory makes little sense when you treat space as space instead of land or coastal waters with fishing rights and oil fields.

Fighting over free passage between system also makes no sense in the hyperspace rules that I have already established. (And which I like.) A single cargo ship takes regular charted hyperspace routes because they can't afford to have a course calculated for every jump. But if a government needs to go from System A to System C on a route that goes through System B, and access to System B becomes in some way closed to them, then it is much easier, safer, and cheaper for them to get a new route calculated for all their military and commercial traffic than to send ships into battle to secure System B.

System Fleets are primarily security forces that fight smuggling and piracy, and to protect the homeworld or colony from being raided and plundered. If a planet is undefended, then sending a small fleet over to shake them down and carry away valuable good would be pretty trivial. But even a modest defense force would make such a raid very costly for the attackers so that the spoils might not even cover the damages they take in the process.
Enslaving a planet instead of conquering it and deporting the population could make it profitable, but that generally gets you a lot of diplomatic problems with the interstellar community. It's rarely worth it.

As interstate warfare goes, everyone has fleets to prevent everyone else from raiding and enslaving them. And usually they are a very good deterrent. Though of course, people constantly make stupid decisions for stupid reasons and fighting between fleets does happen. Usually it takes the form of one state sending a small fleet into another system to intimidate them for diplomatic and economic concessions, and they overplay their cards, and someone starts shooting, and suddenly you have a cruiser shot into burned out wreck. Since there is no faster than light communications, fleets at home can take several hours to get new orders from their government, and for fleet in foreign systems this stretches into days. So commanders act on their own innitative from their own perspective, and might see things different than their government at home. Defending fleets fighting at home are always at an advantage, and systems need their fleets to defend themselves, so more often than not things remain at a single incident or skirmish, which forces the government to reconsider their strategy. It very rarely turns into full size fleets being send to attack other planets. Really big powers, like the Giants or some Red states can better afford it, especially against very small powers who don't have much to defend with, but it's still rare.

Typically, a saber rattling flotilla consist of one or two cruisers with two to four destroyers. That's pretty good to look scary, but if the defenders are able to move a much larger force against them, there is little option but to back off and retreat.

Most combat is pirate and smuggler hunting. This is primarily done with corvettes or frigates, if a system has them. Every third rate colony has at least a handful of corvettes that can deal with most pirates. Sovereign planets and larger colonies even have their own frigates to pursue pirates that are based in neighboring systems. And if things really get out of hand, colonies can send a message to their governments to send a destroyer to deal with the situation.

I think next post might be on ship types and fleet doctrine.