they need to clear the codes to make sure the evap code does not come back.
if you did not tell them to fix the catcon code then they won't admit to fixing it because they do not want to warranty the work. But, if the mechanic is in there fixing something else and he happens to see a quick 1 minute fix like replacing a connector that got unplugged, most of them will do it and then just not record anywhere that they did it. That way, if the quick fix didn't fix anything the customer doesen't know the difference, and if it did fix something then the customer is going to be happy. (and if it breaks a week later again, then the customer won't want it fixed for free)