Tier 3 model of web servers and web browsers

So I have started reading basics about computer networks and a newbie who started college 1 year ago. So I came across this tier 3 model of web servers, I was wondering what's the difference between presentation servers and business logic servers they both seem to have the same purpose! I mean what business logic server can do, can't a presentation server do the same? It seems like the presentation server has no other function rather fetching files from the business logic server! And another question when an HTTP REQUEST is sent what exactly is the response brought from the webserver? What are computations done by the web browser before displaying the site to us? Can someone explain in layman's terms? Thanks in advance