Not OP, but for me it's a longevity thing. With a user-replacable battery, about 2 years after purchase I can simply order a compatible battery, pop it in the phone in a one-minute procedure, and carry on for another 2+ years. The battery will wear out, there's just no getting around that, but today's phones are so powerful that the rest of it isn't necessarily obsolete by that point. Especially not on Android, and especially if you aren't using your phone to do heavy tasks like gaming and media constantly.

It's not impossible to get by without one, but personally I don't have the soldering skills to replace a modern battery (parts are just a bit smaller than I trust myself with) so that's going to be a repair charge when my current phone's battery wears down in a year or so. No reason to replace the whole phone at that point, it still runs great and does everything I need it to do. Why waste the money?

I swap it out once a day, and charge the extra batteries in a charger. This way I can carry backup power that's relatively small compared to most general battery packs. Also I'm putting less strain on my USB-C port, which I've had fail on other phones. Lastly, some people advocate doing it this way because fast charging tends to create a lot of heat, and that heat might damage the phone.