But it's not free, is it?
Not really no. To get the full features you need either a subscription or a one off fee. The server software is free. And you can use it as a local catalogue for local media or to transfer files for free.
You can also get plex server working with kodi (which is also great), which is free, but its a fair bit complicated and not uwp (so it'll work with a tablet but not a phone).
So yeah, not free. 4.99 usd/month or 40 bucks a year Its a good option though for multi-device homes with a media collection, or just collectors in general. There's apps for every platform which helps.
For non-collectors, there's always Netflix. Also apps for every platform. Although that doesn't share all your personal content like photos.
It has the advantage over media sharing devices etc, wireless hard drives, of being accessing remotely (ie away from home). Although technically one can stream from onedrive too, personally I'd never both outside of a few albums because without fibre cloud is a drag for me.