The mortar coverage required for large format tiles is 90%+ so you want to do everything you can to help get better coverage. So if you were to pick up a tile while installing it, the back should be at least 90% or more covered in mortar. Back buttering is when you add thinset to the back of the tile using the flat side of your trowel as a bond coat before laying it down to install on the bed of mortar (which should be combed with the notched side of your trowel so it has ridges). Back buttering tiles will help the bond strength of the mortar, fill the gaps on the backside of the tile, and can help fill any warpage on the tile. In our opinion, this extra step is worth it when laying large format tiles.