Allogrooming refers to one cat grooming another. Between adult cats, especially if they have grown up together, mutual grooming is primarily about cementing friendly relations with each other and not really about mutual hygiene. This is why domestic cats like it when we tickle and/or rub behind their ears or in other inaccessible places.
One of those inaccessible places is under the cat’s chin. It is perhaps the most inaccessible place for a cat to groom themselves. Therefore they find it particularly pleasurable if their human companion strokes or tickles them under the chin. For the cat this feels like being groomed under the chin which is a place that they have never groomed so it makes them happy.
In addition, as mentioned above, it is an important part of the bonding process between the human and the cat. This also is pleasurable for the cat and, of course, for the human because she is providing pleasure to her cat companion, which is something that all cat lovers like to do.
Incidentally, when a cat grooms himself it is referred to as ‘autogrooming’. Kittens experience allogrooming immediately after being born when they are vigourously licked and cleaned by their mother. This may indeed be another reason why cats like to be stroked under their chin because their mother would have groomed them there. Happy memories.
P.S. Both ‘allogrooming’ and ‘autogrooming’ are words used to describe the behavior of animals other than cats.