You can write messages in it.
You can pretend chunks are large cookies
while your kid looks on amused.
Best of all, the mud in this particular canyon on this particular day (Buckskin Gulch, near the confluence with Wire Pass, at the end of June) exhibited a range of viscosity that was just amusing when impacted with projectiles. Crazy clay impact craters are fun!
Alas, certain parties didn't want other parties to get back into the truck fully encrusted with mud, so we stopped short of wading through the potentially waist-deep pools of the stuff.