Sometimes, it's nice to give people an idea of what to expect before entering your home. Maybe it's someone new coming over for the first time, or maybe it's the same friend who's been barging in and stealing food from your fridge for 20 years. Either way, it's nice to give people a bit of a laugh.
Instead of buying a doormat that just says "welcome" like all the rest, why not use one that has a little more personality?