this is not a practical answer but it's an actual answer..

Straws are useful for drinking lemon water, and maybe other drinks which have the wrong kind of acidity that can damage tooth enamel.

The straw can help to prevent the liquid from making contact with the teeth as you're drinking.

So, there's 2 different untested theories there, I am careless spreading around without having tested either of them myself, because I have unusually strong teeth, which is an uncommon but not rare thing.