You probably don't need us to tell you that a proper DSLR camera is better than the camera on your smartphone. But if you're curious about how much better it is, the folks at Bammo have a (slightly NSFW) video that explains everything you need to know.
The video is a little over a year old and uses the iPhone 5 for comparison, but the general points remain true even today. A DSLR can take much better photos in low light, can take much better action shots, and can produce RAW images that are much better for manipulation. The iPhone doesn't suck if you use it right (and the Lumia cameras are even better), but a DSLR still produces noticeably better shots. If you haven't tried out a good DSLR or mirrorless camera before, it's worth knowing that the differences can be huge.
Check out the video above to see Bammo's explanation. Note that the video has a few slightly NSFW moments, but it's one of the best videos we've seen on the subject.