well, you can exchange cp's for credits... They can also be exchanged for ships, and vehicles... They can not make you "powerful" except for the fact that you might have a better ship than some people, but someone else is bound to have something better... They are meant to be extra things for the player, rewards almost... Not really a game critical use, except for roleplaying. If you want power though, join a faction.
CPs can offer some nice bonuses, but you can't get anything with CPs that you can't get the equivalent with credits, except for custom images and items. So if you want your stuff to look nice, and have some non-functional RP items, then you need CPs. Otherwise, credits will do you fine.
Or to put it another way:
Click the daily banners to earn CPs and promote SWC, but don't rush out to "buy" more CPs with real money because you worry that you'll need them.
(But please feel free to donate if you do have a little extra cash, as this keeps the server running, and so on.)