Because most people plan "Events" on the weekends, so if you need to travel and don't want to be rushing there and back, its a smart idea to take Fri/Mon off so you can spend each of those days traveling for whatever was planned and have time to relax and get into the groove of whatever is going down.

If my buddy has a wedding on Saturday two states over, I'll drive friday, party saturday, sober up sunday, and then go home monday. And then I am not limping into work exhausted from a long trip I had to push through.