Giving a full readback and then stating WILCO is redundant (you just told me you got it (& you need to comply unless you tell me something different) by reading the pertinent instructions back).
"Wilco" can be used for any requests not requiring a readback.
and trying to think of an example: maybe reporting passing a lvl or geographic location which technically has no bearing on your clearance etc. but will facilitate the clearance of another.