Controller .... Procontrol in my particular case.
Serious mixing without a controller of some kind is virtually impossible imho. To be able to put your hands on several faders at a time is a must. Direct access to parameters like pan / volume / sends etc .... just isn't the same with just a mouse or a trackball. Controllers like Procontrol / C24 and Icon, be it in Protools' case enhance your workflow tremendously.
Track arming during recording sessions come to mind ... automating tracks and parameters ... pure software commands like grouping / zooming / new track / copy / paste / delete / ... all with dedicated buttons (although most of those have keyboard shortcuts) ... it just isn't the same without them.
Working a daw without a controller ... kinda like driving a car with square tires.