Well of course a DSLR will be better than a phone camera for most things, since you can control more features. That's also the point of OpenCamera; get as much controls as you can back, compared to the stock app that might be lacking. It wont do miracles if your phone camera is not good to begin with.

I've found OpenCam to be useful because of the various optional features ( onionskin, levelmeter, locking settings ). You can also set the bitrate/size/duration of videos, etc. Lots of useful stuff.