Swells, especially if you’re playing country steel-style guitar. I use it for modulation or synth effects that I run parallel with my main signal to swell those in as well

Placement physically, as has been mentioned. They could also be useful for gain staging, if they’re somewhere in the middle of a chain. Certain effects might sound different depending on the input level.

Mark Knopfler has used one forever.

I use it for gain control during parts, for when I'm actively playing and cannot act the volume pot but I need the gain to go up or down

To create swells easier than using a pinky on perhaps a distant knob? Or where you might want the volume. But not the gain or also not to adjust the sweet spot the guitars controls are at? Volume pedals are pretty useful tools.

I switch from Tele to Strat, rinse & repeat, during shows. Guitar, VP, Tuner pedal, pedal board, Blues Jr, Shure 57, stage box to mixer. No noise when switching guitars as I kill the volume on the VP to make the switch. Volume on guitars & VP full up while playing. Volume on Blues Jr gets set to make our sound guy happy. (I play rhythm.) It works for us.