I know everyone remembers Willis at his best getting a dozen tackles a game, but in case you haven't been following the evolution of the defense and his role for the past two years, Willis has been asked to drop back into coverage more and more often. So when he's "standing around" in the center of the field, he's in man coverage on the tight end or running back to make sure they don't slip out for a late screen or quick strike.
Since Bowman has inherited the role of "The Tackling Machine", Willis' responsibilities have gotten more complex. Before the St. Louis game, we had the 2nd best passing defense in the league. I think you can credit Dashon Goldson and Patrick Willis for the secondary's improved play, along with Perrish Cox as the 4th DB (big upgrade over Tremaine Brock). I'm sure if you put Willis back in his old role, he would excel. But that's not where we need him, not with the emergence of tight ends in the modern offensive game.