Well you can of course connect all sorts of external storage loctioans to your Nextloud. But keep in mind that Nextcloud was primarily designed to replace classic file shares and network drives and not to act as a web front-end for them.
Well it’s not useless for me. I can upload and download files, and even edit documents directly in the app. That’s pretty much all I expect from an app like this. It basically works the same way as client apps for other cloud services work, like for example the Google Drive app. However, as I understand it, the devs are not generally opposed to a synchronization feature. But as it is often the case with OSS products, it needs someone who is willing to do the work. 
Here’s a first-hand explanation: Syncing with Android not working. How's this meant to be? - #13 by Andy