1. Before open access
You write a paper. It is available for free from your website, SSRN, IDEAS etc. It can easily be found at Google Scholar and read for free.
The paper gets accepted at a journal.
The publishers make you take it down from your website. They publish it on the journal website. They charge large fees to universities for access. People outside universities cannot read it. (Luckily, early drafts are available at the university websites where you have presented the work.)
2. Open access arrives
Information wants to be free! Activists demand that journals provide articles on open access so that anyone can read them -- especially if the government has paid for the research.
The government agrees. From now on, government-funded research must be open access.
3. Now with open access
You write a paper. It is available for free from your website, et cetera. It gets accepted at a journal. The publishers offer you an option: publish it as normal, (i.e. stop people reading it) or publish it open access so that everyone will be able to read it. The fee is a snip at £2000. Your research is government-funded, so you (your university) have to pay the fee. Or, if you want it to be included in the next Research Excellence Framework assessment, again you have to pay the open access fee.
Your research is available for free on the journal website. Anybody can read it. It is easy to find at Google Scholar.
Everybody is happy!