Because your internet traffic moves to and from your devices (computer, phone, tv, tablet) through your ISP, they can see where you go online. An ISP can see what sites you visit, how long you’re on them, your location and information about your devices. An ISP may not know the specifics of what you did on those sites (like what you bought, searched for or read) thanks to encryption, but they could make inferences about you based on the sites that you visited. That personal data can be used to create detailed profiles about you. Why would ISPs do that? In short: this data is valuable.