There are many different bots which require the User to set a timezone for specific things, like a custom chat-system, a calendar reminder thing, etc.
So it would be beneficial to already know the current timezone a User has, to take this extra burden from the Users themself.
I'm pretty sure this would also help the bot developers themself to determine the time of day for a user, which could be useful for features like a anti-ping notice ("Please don't tag <user>. They're probably sleeping already.") or similar.