Another good reason to use this (a tip I could have utilized today but never thought about it) is if you are sanding surfaces that are not completely flat, but are supposed to be- it helps you find the high and low spots.

Actually, for what it’s worth, sanding does not open the grain. Sanding essentially burnishes the wood pores, and the higher you sand, the more they close up- which is why it is advisable to never sand above 220 (even, at times 220 can push it) when you’re going to stain wood (Various wood types all differ on this…