The assembly optimization for SSE is the code that occasionally pushes many AMD machines into unstable conditions. The parameter -forceSSE simply gives those who have demonstrated their machines are stable with SSE operations to regain the extra 15% speed boost that they had before the default was changed to 3DNow optimizations.
There are 3 types of code in the PC version of Gromacs: Unoptimized, 3DNow+, and SSE. How you invoke them was changed depending on the flags and the client version, but the fundamental code is still the same.