My impression is that people wanted to make use of API's from third parties which would have been prevented by the same origin policies. CORS allowed them to r

My impression is that people wanted to make use of API's from third parties which would have been prevented by the same origin policies. CORS allowed them to relax that policy for the API's and services they wanted to use w/o totally removing the security against cross site scripting that the same origin policy provides. Having to proxy all requests through a server would have defeated the purpose of the new breed of paid APIs who's end goal seems to be to allow you to build a complex web site w/o having to wade into the details of maintaining the backed servers you would have traditionally needed and all the complexity that entails at scale.