[est31]

You don't always have a clear line of sight. E.g. when you communicate with a satellite orbiting mars, the planet will get in between for a period of time.

Furthermore, even if you have clear line of sight, repeaters on the way can be helpful because the inverse square law even holds for lasers (the light isn't perfectly parallel, there is no such solution to the maxwell equations (outside of an infinite plane emitting light perpendicular to it), instead it's a gaussian beam).

Last, you only have a limited amount of spectrum you can send to or receive from a specific point in the sky. Beyond that, it gets hard really quick. If you have repeater stations far away so that they can be targeted from the distance, you can increase the maximum bandwidth. This is mainly a concern once the interplanetary bandwidth is maxed out.