Dipping harmonicas in water

I’ve seen Neil Young and Bob Dylan dip their respective harmonicas into a cup of liquid (I assume water,) before playing them on stage. Why do they do this? Also, does this harm the harmonica? Does it help the reeds in some way? It doesn’t seem like it would. I’m sure a $20 harmonica is not a big deal to them, but I don’t want to go ruining all of mine by dipping them. Help. I really want to dip.

My first guitar teacher also taught me some rudimentary harp skills. He said that a worn out harp could be fixed up by dipping it in water or beer. I don’t know why this would work – the sound-producing parts of a harp are metal reeds, and they shouldn’t be affected by moisture.

The wood body harps will swell a bit and produce a bit more ‘boxy’ sound; and it also makes the harp louder.

It also tends to trash the wooden body harps, what with repeated shrinking and drying. If you play your harps often and loudly (cross-harp or ‘blues’ style) this is less of a concern since they get trashed fairly quickly anyway.