Not sure ... maybe just to look cute?
Just joking.
There are no dumb questions ... just often regrets they were not ask. That's why we are here ... we all have experiences and/or research, that someone else does not have, and we share it all to benefit the chinchillas.
Their fur is very, very fine, as we all know ... I would think it might be that a bit of loose fur might tickle their nose and they wipe it away, same as we do. Their noses and whiskers are extremely sensitive ... something just touching them can tickle.
On the serious side, if a chin is
not just wiping it's nose, but also it's mouth ...
and it is drooling or has a runny nose ... it's time ... just for safety ... to get their teeth checked out. Possible malocclusion, if caught early, is easier to treat.
Does either of the eyes look watery?
Jo Ann