Just a guess, but it could've been a way for them to determine if you were about to paste content into the address bar. I worked for a company that did something similar. It would compare the clipboard to the contents of the input field. If it was a URL and they matched it would assume you had pasted a URL and would take some extra actions based on that.

It wasn't until the OS started alerting the user to this clipboard read that it started to look very suspicious.