We are implementing our first SharePoint installation (2010 version). Our
initial inclination is to have one web application and under it 3 site
collections (published intranet content, teams, and My Sites). However, when I
look at MS TechNet, their diagrams seem to have separate web applications for
each. Also, they have a separate site collection per publishing department or
team. We were planning on having departments and teams as sites only.

Can someone tell me why MS would set up things this way? Is this the way it
should be for big companies. I'd like more information on the criteria for
deciding what should be in a web application vs. multiple web applications and
one site collection vs. multiple site collections.