It's true that effective range of weapons can only go so far. But "so far" is tens of thousands of kilometers, which is
pretty far.
The word you're searching for is inertia (or maybe force). Although if there was no inertia.....well, it's hard to put what it'd be like into words.
Gravity is just another force. In terms of the planet earth, it's a force that's pulling (although every force is really a push, never a pull, but for the sake of simplicity...) everyone to the ground. If you tossed a ball upwards, it would come back down.
In space though, the force of gravity is so negligible that if you toss a ball one way, it's bound to keep going for who knows how long until something stops it.
That's why combat range in space will be very very large. It doesn't take much force to accelerate something in an environment where there is very little other force acting on whatever you're trying to propel at however-insane-velocities-you-want. And since any object in deep space is likely to keep going that way, you don't need any more force to keep that projectile going as well.