Monkeys evolved as fruit eaters. Banana is fruit. In fact there is so much vitamin C in fruits that monkeys (and their descendants the apes and humans) have lost the ability to make their own vitamin c genetically. Other mammals can make their own vitamin c, which is an important precursor to making collagen, an important structural protein for skin. Vitamin c shortage will cause scurvy and death. That is why monkeys, apes and humans must eat fruits or other kinds of food that is rich in vitamin c.
Monkeys, apes and humans have a sweet tooth, meaning we have taste buds that detect sugar (there is a lot of sugar in fruits) and our brain feels pleasure when we eat sugar. Bananas contain sugar and other nutrients, including of course vitamin c. Therefore bananas taste good to monkeys, and they get calories and nutrients from eating bananas. It is good for their health and and they get sugar high. So, why not eat bananas when they are available.