For me, it depends.

I used to work at three locations around the city, and didn't have a locker at any one of them. So I had to carry that day's clothes and lunch with me. A trunk bag was just fine for that.

Now I work my entire week at one location and I have a locker. I use panniers on Monday to haul in the entire week's worth of work clothes, lunches and snacks. On Thursdays (four-day work week) I use the panniers to haul home the laundry. In between I can ride any bike I want without having to be concerned with hauling anything.