what does a dual camera do?

What is its purpose?

Gimmick or must have?

It makes camera quality so much better. I have this on my LG V20 H990DS and I consider it very worth it if you can afford a little extra.

The wide angle is my favorite. LG really are perfecting it with every iteration.

Zoom is useful I suppose but I hardly ever need to zoom. They also allow portrait mode but google has shown you don't need a second camera for that.

Black and white camera seems a bit like a gimmick. Not sure it has any real benefits

So yes second cameras are useful to have an extra option but I wouldn't call them necessary. I'd much prefer they work on getting larger sensors into the main camera.