I've read that some birds eat grit and it is necessary for digestion of food. Is that true? Why would that be the case, and if some birds need to eat it, do all of them? I thought all birds had basically the same parts. Thanks!

Grit is eaten a lot by seed eating birds. Basically it accumulates in the gizzard and helps to break down the tough seeds by abrasive action (remember birds have no teeth) which makes them more easily digested.

All birds have a gizzard, some other animals do too (see https://en.wikipedia.org/wiki/Gizzard) but not all birds feed on things as tough as seeds so many don't need to ingest large amounts of grit.