Redundancy. I have two independent firewalls, each separately routing traffic out through two totally independent multi-homed network connections (one cable, one DSL, please god somebody give me fiber someday) that both firewalls have access to. For awhile was thinking of replacing the DSL with starlink until Elon turned out to be such a pile of nazi garbage, so for now DSL remains the backup link.

To make things as transparent as possible, the firewalls manage their IPs with CARP. Obviously there’s no way to have a single public IP that ports itself magically from one ISP to another, but on the LAN side it works great and on the WAN side it at least smooths out a lot of possible failure scenarios. Some useful discussions of this setup are here.