Well, one of the biggest reasons to have a VPN is just the privacy. Your ISP spies on you, and sells your browsing data to corporations like Amazon. A good VPN will also block ads, trackers, and malware, which helps keep you safe while browsing. You might also be on an ISP that throttles some types of traffic, such as streaming media.

A VPN, as long as it's set up right, prevents anyone from seeing what you're doing, even if they have access to your traffic. And if you're in the US, there's a strong argument that being free from casual intrusion into your internet habits by corporations and the government is a Constitutional right.