I was never told to tap the mat when I started. However I often do it now involutarily when it is a fast/high/energetic throw which is mid way between a break fall and a ukemi. In fact I get a weird 'two tap' feeling often. I think the first one is when my body is still unsure how hard the fall will be, and the second one helps me get spring up from my ukemi.
Getting to tap the mat will probably make you aware that sometimes a throw is too hard to ukemi out of and you must be able to adapt immediately. However, a proper slap on the mat with the ukemi could be jsut as useful.