I'm a guy so I don't know everything a female SB gets from having a premium but there's two advantages I do know about.
One; you can see which SDs are premium. Since only premium SDs can read messages from SBs this keeps you from wasting time contacting guys who are just on the site and looking and can't read your messages. Guys who don't have premium can actually send a certain amount of free messages but they can't read the replies. So don't be fooled into thinking a guy is premium just because you receive a message. Many SBs get discouraged because guys don't reply to them however they don't realize most guys on SA can't even read their messages. I'm pretty sure most guys on SA are non paying members.
Two; As a premium member your account shows up on a guys search list regardless of his settings. I know this because sometimes I search for white women only and yet I'll see the profiles of black premium members.