DNS is a service that converts easy to remember names like google.com or youtube.com to hard to remember ip addresses so you don't have to. We find it easier to remember names than strings of numbers. So usually your isp assigns their own dns server for you to use automatically and most people don't think about it. Sometimes a DNS server can go bad or become slow and people use services like google's dns services to speed up access to the sites they use. Google could use dns look up information from your computer to know what sites you visit even when you don't search google.com for it. I don't know if they do or not but I was thinking, if I was google I would probably do that. Its not that google needs your browsing habits to find web sites but it can use the querys it recieves to measure the popularity of web sites or web pages. As to why the current interest in DNS I'm not sure.

A lot of trust is in DNS and a lot of people don't realize that an evil dns server can make your browser think you are going to good sites but could actually be an imposter.

Other annoyances are NXDOMAIN (non existant domain) querys that are hyjacked to serve you ads.

Controling DNS servers is a great way for a nation state to censor communication. They could take any site they wish and forward your browser to an automatic service that logs you with other "wrong thinkers". Or just resolve the domain name to null reference.

Speed, security, privacy are things to consider for DNS. But like a lot of things on the internet, it all about trust...