How are satelites useful?

[PvK]

Other good things about satellites:

1) Can't be hit by several kinds of direct fire weapons.
2) Can't be hit by seekers.

[cascalonginus]

Arent Sats limited by inherent cargo space on planet?

[Suicide Junkie]

Not if you launch them into orbit Then all you have to worry about is the "units in space" game limits.
You can launch extras beyond that limit while in combat if you devote planetary cargo space to extra sats. However, the strategic AI will not launch sats in combat, you'd have to do a tactical combat.

In general, launch sats, and fill your planet with Weapon platforms.