In short, it's about search ad revenue (placing google or whoever as first choice for search) and (possibly) selling and giving out more data (bookmarks, URL history (supercookies, etc), cookies, login usernames, etc). I know that firefox for Android offers no option to not remember form data (it does so by default). It can be assumed some of this data is sent off to homebase.
With the various "hand-holding" security features found on the desktop browers they also monitor basically every website you visit to make sure it's "safe". By doing so, the developers know a lot about their users. Profile data of millions of users is being built up and sold in backroom deals, basically.
Most of the major tech companies know that most people have long since stopped caring. They want free. I heard Firefox will get integrated ads soon.