Being the most heavily fortified system in the galaxy Sol is very busy. Titan is home to the Grey Knights. Pluto has a listening post as an early warning system. There are lots of star forts, the Phalanx itself is often there. Artefact 9-Kappa-Mu is a thing.
http://wh40k.lexicanum.com/wiki/Artefact_9-Kappa-Mu The moon is a highly strategic important position, holds a naval base and is
HQ to the Sisters of Silence and Battlefleet Solar. Most of the rest are seemingly used for resources and mining. And lots had uses during the 30k period.