I am starting my 4th tile job and have always used glue on both floor and wallsl. Is there any any good reason to use morter on floors compared to just glue.

I answered a similar question here:

https://www.askmehelpdesk.com/interior-home-improvement/difference-between-ceramic-tile-mastic-thinset-313048.html

But, mastic should never be used on floors and for walls in wet areas, you should also also use thinset and not mastic. For those that say that thinset doesn't keep the tile from sliding on vertical surfaces like shower surrounds, then it's obvious they've never tried any of the lightweieght nonsag mortars such as my favorite Tec's 3n1 or Laticrete 255 or Custom's megalite. Premixed thinset is mastic, by the way. Manufacturers just add sand to it and charge a ton of money for an absolutely inferior product to make diyers think they are using the proper materials.