Most important of these was the fact that the taxidermy specimens we used for the 'animate' object category were relatively fragile, and if you've ever seen footage of a wild chimp who has just captured a colobus monkey, you'd see why it would not have been wise to let the apes handle the objects. There are at least two reasons for this. One, is that grooming serves to rid the chimps of parasites, and it is difficult for them, or anyone, to pick small bugs off parts of your body that you cannot see, making grooming by others very practical. But grooming also plays a social role. Grooming is a way of communicating, making friends, firming up social relations, and making up after fights.
Chimpanzees are a very social species, and grooming is one technique in which they relate socially.
As to why they put their hands up in the air, your guess is as good as mine. Although all chimpanzee groups groom, only a few put their hands up in the air while they do so. You can think of this as similar to different forms of greetings among humans. In Japan, for example, a bow is appropriate, while in the West we shake hands. Bowing versus hand shaking reflects differences in culture, and similar claims have been made for chimpanzees.