To cycle in fresh perspectives, and limit the power that can come from decades entrenched in office. People like Pelosi or McConnell are so powerful in part because they have been in office forever, have a network that is both deep and wide, have lots of people who owe them favors, etc. If you set a maximum age plus a lobbying ban, that enforces an end to that at some point.

I'd be in favor of a two-term limit personally, for many of the same reasons. If it's good enough for the President it should be good enough for other offices. I don't think political office should be a lifetime career. It attracts too many power-seeking, controlling personality types. We don't need those people around forever. Too many people in office have no private sector experience at all, yet they are making the rules that the rest of us live by.