Cross-origin resource sharing is a mechanism that allows a web page to make XMLHttpRequests to another domain (from Wikipedia).

I've been fiddling with CORS for the last couple of days and I think I have a pretty good understanding of how everything works.

So my question is not about how CORS / preflight work, it's about the reason behind coming up with preflights as a new request type. I fail to see any reason why server A needs to send a preflight (PR) to server B just to find out if the real request (RR) will be accepted or not - it would certainly be possible for B to accept/reject RR without any prior PR.

After searching quite a bit I found this piece of information at www.w3.org (7.1.5):

To protect resources against cross-origin requests that could not originate from certain user agents before this specification existed a
preflight request is made to ensure that the resource is aware of this
specification.

I find this is the hardest to understand sentence ever. My interpretation (should better call it 'best guess') is that it's about protecting server B against requests from server C that is not aware of the spec.

Can someone please explain a scenario / show a problem that PR + RR solves better than RR alone?