What is the purpose of an article repository I was asked by a publisher to choose a repository where the article to be published will be uploaded to. What's the point of this if the article is already available on the journal website?

Can someone please explain what is the purpose of a journal repository such as PubMed, Europe PMC, PMC?

What are the pros and cons of uploading an article on a repository? Is it desirable to use more than one?
I was asked by a publisher to choose a repository where the article to be published will be uploaded to. 


I've never heard of that before. Publishers usually want exclusive distribution rights.


  What's the point of this if the article is already available on the
  journal website?


You can update the repository version to make corrections, should they be necessary.


  Can someone please explain what is the purpose of a journal repository such as PubMed, Europe PMC, PMC?


To disseminate research. 


  Is it desirable to use more than one?


One suffices. Increasingly it seems that repositories don't want you to upload elsewhere.