I'm not sure the exact idea here because they said something about retrieving PDF and JSON data, but a big reason for not doing this is normally that some calcu

I'm not sure the exact idea here because they said something about retrieving PDF and JSON data, but a big reason for not doing this is normally that some calculation or whatever takes a lot longer than you want a HTTP request to take. So you effectively push the work off, and then the user comes back at a later time and something has been processed and they can see the result.