Why source a file?

The first time you either see a . (dot) in a line of shell programming or someone tells you to source a file, you're first reaction is "huh?"  I did it, and I even heard it in a colleague's voice on a phone call last week. My colleague was by no means a junior IT person, but I could still hear the confusion. I decided to try and clarify for some folks so they don't struggle as much as we did.WizardsOwl