If screen readers do that, why? That seems useless even when those characters are used for their intended purpose.

Yes, I know people do things like that (mostly on Twitter, it seems), but I wouldn't recommend it; it'll interfere with searchability, copy/paste, etc., and generally make the text less interoperable. The mathematical alphabets in Unicode are intended for specific technical uses where the typeface is an inherent part of the symbol's identity, not for styling normal text.