Let's say SBD is worth $0,95 and you convert this to $1 worth of STEEM you basically earn an extra $0,05 of STEEM compared to the internal markets.
I was actually asking myself question “why would anyone do that?”. This completely answers my question. With bigger amount to exchange it could be a very good deal. Can you imagine for SBD to go much lower and Steem in reverse to go much higher?