It's always important to get the walls vertical and straight with no waviness laterally or up and down. Otherwise it means a lot more glue and a lot more time to maintain a straight line and a good look when tiling.
Floors also need to be level. It can mean a much longer and expensive lay if the floor level is going the wrong way.
If it is straight then all I need to do is to drag the glue over with the trowel and lay. Otherwise it gets thicker and deeper and much slower! Time is money.