Ask HN: Why should any serious business use a third party API?

I really need your help in figuring this out. Lot of established companies have APIs that we can hook into. It enables us to access their data which coud be beneficial to our app/business etc. However, if our business is for-profit and using someone's API, aren't you most likely to have confict of interest or violation of TOS etc. ? so why should APIs exist at all to be used by businesses that charge money. Whats the point of APIs? put some cool data on my personal blog ?