Or even porting accidents. If anything goes wrong when you are switching carriers and intentionally having your number ported to the new carrier, you could be left with a non-working number until you get it resolved (which can be difficult because your phone is no longer working...).

And even if nothing goes wrong, it can take up to a day for the number to port in the US. That's a day without being about to login to anyplace that requires SMS 2FA.

A TOTP app on my phone, on the other hand, works fine regardless of whether or not my phone number currently works with the SIM in that phone.