Some of the ISP routers do not allow you to put your VPN on the router or block changing the DNS. So for 98% of users the ISP is fine and no gain.
You have to have a reason to use your own router. I can't give it to you.
I currently use Windstream and they provide a combo modem/router that is doing OK. No issues with Wi-Fi or streaming. My question is, would I gain any thing by using the Netgear Nighthawk AC1750 Smart Router Model R6700v3 instead of the router side of the Windstream box.
Tony

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic