I think it is less common now, but ISP routers on average used to be trash with issues — bufferbloat, memory leaks, crashes — so a number of people bought a higher end router to replace the ISP provided one. Mostly tech savvy people who were not necessarily in IT.

Nowadays my ISP just uses dhcp to assign the router an address so you can plug any box into it which talks ethernet and respects dhcp leases to be a router which is nice, albiet 99.9% of people probably leave the router alone.