In addition to the other answers many of the ISPs provide their hardware on essentially a rental model. The benefit to that is that if you complain loud enough they'll replace it on their dime: the detriment is the same as with any rental though, typically they overcharge you versus what you would pay if you bought it yourself. Your mileage will probably vary, but I've certainly saved money in the long run owning my own equipment, even paying for my own replacements versus storm damage and similar.
Another consideration is that often they will try to use the fact that you don't use their hardware as an excuse to not go further in troubleshooting since they "don't support 3rd party hardware."
It's fun when you get the opposite too and they follow their support scripts regardless of your 3rd party hardware and above average knowledge of the subject:
Support: On your hardware, do x.
Me: Did it already. It reported "Y".
Support: Please do it again, following these scripted steps [what don't match my actual hardware]...
Why not?