At the moment, it’s just an ad, really nagware, for users to sign up for a premium OneDrive account. But Microsoft’s nagware behavior this past years is causing some serious concern among users.
There are also third-party File Explorers like Total Commander which don't contain ads. But, you may need to pay some money to unlock all the features, and more advance features. I'm guessing, more people will be switching to third-party file explorer programs instead of using Windows Explorer. Windows Explorer is not very good in my experience since it seems to crash a lot when I used it in Windows 98, Vista, 7, etc.
The fact that you even have to turn off advertising in these places in Windows 10 (the start menu, file explorer, office? Any others I've forgotten?) is obnoxious, as if people don't get enough things advertised to them in a day. And from their OS no less... It's a disgusting practice. (Especially now that they're charging for copies Windows 10 to basically be advertised to? How about no.)
As for explorer replacements, Multi-Commander is pretty good. It's free and is decently powerful. Moreso than Windows built in one.