Microsoft is the main reason Bill Gates became wealthy, and we know how many people around the world are using Microsoft as their operating system on their laptops and computers, right? There might be some that are using Linux or Mac, but a huge percentage are using Microsoft, so it's like how he helps other people because at the end of the day, we are the reason why he became rich (since I'm also using Microsoft as well).

Another one is that, he can't bring those wealth when he dies so might as well give it away to those people that needs those wealth. Let's also not forget that Bill Gates is a prominent philanthropist as well so throughout his life, he's helping charities here and there. With those reasons, I guess that's enough already to say why he's giving away his wealth. This isn't the case for most of them though because there are some who are just giving it towards their family members, and not on other people.