How can bypass surgery help after someone has already had a heart attack?

CABG after MI: Bypass is often done after heart attack. Many times there is still "viable tissue" that is susceptible to further insult(and worse outcome) that may be saved by revascularization. Also, the heart attack may involve one artery in the heart yet the other arteries have disease that must be bypassed or there may be another heart attack with a much worse outcome.

Prevention: Prevents another heart attack, by bypassing the blockage in the coronary arteries ...

Yes: If there are multiple blockages or blockages near the origin of the main coronary arteries, then bypass surgery can redirect blood flow around the blockages so the remaining heart muscle stays healthy, even after a past heart attack. The goal is to prevent future heart attacks.