A dozen different things, depending on what you're trying to do...
- Delay block's In Level parameter
- Gradually morph from one amp-cab signal path to another (i.e. clean to lead)
- Depth or mix level of certain effects blocks (tremolo, chorus, flanger, phaser, rotary, etc.)
- ... and many more.
I have three expression pedals (two built-in on my X15 controller, plus a third one). I always have the third one assigned to a volume block placed before the delay and reverb blocks. Of the other two, one is almost always assigned to delay in-level. The other one varies a lot from preset to preset: wah, or any from the list above. With this minimal amount of "stability" in my presets, it's not too hard to remember what does what (two of the three expression pedals almost always do the same thing).