This may vary by school, but at my school, students with outstanding participation may receive a one-step bump in their grade if they are near the margin. So a high B+ may become an A-. One professor said he typically does this for 5-10 students in a 120-student class.
I've heard stories of students who are perpetually unprepared getting their grades reduced, but I think it's less frequent.
Seminars may weigh participation more.
So it's my first week and I am being assigned like 8-9 cases a night to brief. Should I actually be doing that though? It takes like all day to brief that many cases and after that, I have no time for anything else. Most of my professors claim that participation will affect our grades though.