Being part of the 501st is like having gained thousands of new brothers and sisters, (and distant cousins!)
As a good example, I am a member of the Pacific Outpost, and the local Garrison here in Washington, caught wind of me being on vacation here and treated me and my girlfriend to a nice lunch and we met and shared stories. Our detachment leader, Paul, was at the lunch, was kind and offered me a Titan Garrison patch as a welcome gift, as well as an Expert Infantry patch, which I will be earning sometime this year, when I upgrade my FX TK to TM armor.
Additionally you gain access to 501st goodies not available to the public...
EDIT: being in the 501st allows you more opportunities to wear your costume and even do good with charity work.