From the ChipMixer FAQ:
Betting and donate allows you to change possible output value. Even if blockchain analysis may assume you will receive permutation of chips, they cannot predict how much you've lost / won betting or how much you've donated. As with split / merge, you don't have to use it - existence of this option increases your privacy.
Yeah I get it is totally optional, but I think it was just my personal opinion on gambling that made me have that view, since a while back you were able to gamble with skins or chips on a video game. You would deposit your items in a site and they would give you coins to gamble with. It lead to a lot of younger people to start gambling and kinda ruined the game for me.
I really like the idea though, and I see how it makes it a lot more anonymous. It kinda adds a certain enjoyment to mixing, which is usually a pretty boring process to go through, I might try out the site later this week, and I'll leave a review.
My understanding is that using the betting function can allow you the possibility of gaining bitcoins just like betting on a gambling site so you can mix the coins and take a profit.
The donation option allows the user to voluntarily tip the site for their services. Most people should be willing to pay something for this service but this gives you the choice of how much to pay.
I think both of these functions enhance the site because they allow the customer to make these decisions.