Me! Discord does not consider me a real person if I don't have a phone number it approves of.
Users that need special clients (accessibility, hardware, etc.). Users blocked by US sanctions. Users that have been moderated off the platform for something not in your community (account bans even happen accidentally). Users with privacy/anonymity concerns about the data collection (& now ads)—especially the chat rooms that require a SIM card. Users that take their FOSS or otherwise ethical software views or anti-corporate views to heart & want something built on those principle—from wanting to use free software to make free software, to wanting to outright avoid what some now call enshitification where a free (gratis) account clashes with the idea of freedoms, etc.