Chessington's zoo sucks though (for a zoo, there are plenty of far better places to go). I think it could do perfectly fine without it - if the theme park finally had the attention & investment it needed. Probably 80% of guests come just for the attractions. But it is simply a way for Merlin to stick a 'brand' on the park, give shareholders comfort that it won't compete with Thorpe Park and pretend it is Disney's Animal Kingdom in advertising.
Sure they maintain a good image with animal conservation and raising funds abroad, while keeping all their animals in dated, tiny and falling apart infrastructure. Or in Zufari, which is just weird and most the animals died after a couple years anyway.