Simple. It makes a very nice 'helmet visor' when you're riding toward the sun!
I use a 8-10 inch piece that's been folded over slightly on both ends. I stick it to my face shield at the right position to block the sun. Then I tip my head a bit if I need to move it a little.
If I need to move it a lot, I just reach up and move it to a good spot. (Yes, you have to be able to ride with 'no hands' if you don't wanna pull over.)
So why painter's tape specifically? It doesn't leave anything on your visor and it's easy to move and remove. You won't need much, so don't bother taking a full roll on a trip. In fact, just a few pre-made strips stuck to the inside top of a saddle bag or topcase should work for most people. My recommendation is to use 1 inch wide tape. You're going to put it very close to your eyes, so the 1 inch width is effectively much wider than it seems from the outside.
Here's a pic of it in use. Nothing spectacular. Just a piece of tape.