If you're in a fleet, you can acquire fleet marks, which are often required resources to advance a fleet base to the next tier. I'm currently at a Tier-6 New Romulus reputation, which gives me access to assist units, both for Ground and Space missions. You can go out to various skirmish and battlezone areas, earn personal marks and put them towards endgame features like 'Reputation'. There are weapons and equipment vendors there that sell items that can't be found at main faction bases. A Fleet Tailor becomes available at a certain point that provides access to uniform types you can't find anywhere else (that's where I got my Odyssey Dress Uniform which is the closest thing I can find to a TWOK-era 'Monster Maroon' on the XBox, if tricked out just right).
You can purchase extra slots for Upgrades, for example, but only at a certain tier.
The higher-tiered the base, the greater the resources the fleet memebers have access to. Fleets are organizations of other players that contribute their resources to build several facilities, such as starbases, science research centers and other things.
