Because free DNS services know which sites you want to visit and which services you subscribe to, the operators of these services can learn a lot about you.
Some free DNS services make money by creating and selling personal information about their users. This information is used primarily for advertising. For example, it can be used to assess the marketing value of a website by determining its popularity and where visitors are coming from.
Innovations such as DoH (DNS over HTTPS) and DoT (DNS over TLS) can protect DNS requests. However, most free DNS services still don't use DoH, DoT or any other encryption method. Not only can these services see your activity, but anyone can listen in on your connections.