Sandy has the right answer when she says:
It seems as if advertisers have caught on that if they have kittens or cats in their ad, they’ll get the viewers attention, so it doesn’t matter whether the product is chocolate, cars, clothes, toys, or electronics, etc. they’ll use whatever works, and cats are the most viewed online.
You can add a bit to that. In the advert the cute blue British Shorthairs (always a very cute kitten) substitute the McVities biscuits. They come out of the biscuit packet. The kittens become the biscuits.
As kittens are gorgeous, extremely attractive to most people, cuddly, smell nice, reassuring and make us smile they promote the biscuits. The biscuits become very attractive to the eye just like a kittens. The biscuits are associated with something attractive.
I just wrote about cats promoting stately homes in England. Their photos online attract people resulting in more visits to the homes. The cats also enhance the ambience in the homes.