When you say "SQL" are you referring to the Server Enterprise Manager (SEM)? If so, ERWin does a lot of things that the SEM doesn't, and vice-versa. SEM is a tool for administrating SQL Server. That include functions for manipulating the physical model of the database. ERWin on the other hand is a modelling and repository tool. It has functions for maintaining both the physical and logical models alongs with tools for reporting against the schemas. And that's just the tip.

It's worth a trip to their website.

-Mike