The SP/Bandwidth ratio dictates how much data(aka bandwidth) you can use on the STEEM blockchain. I think it was put in place to prevent new accounts from spamming the Blockchain (they would run out of bandwidth fast). But don't pin me down on it. Bandwidth will become way more important when SMT's come out. You will need Steempower to run the SMT's. (the more the SMT is used to more bandwidth you will need).