VoIP phone presence vs lines vs

Where can I find documentation explaining “lines” or “presence” on phones? What I mean is, the concept of lines in the voip world is so different from the old pbx days where you had a seperate pair of copper wire running to each phone for each line. You had to have separate buttons on the phone so you could answer whatever line was ringing etc. Now with voip, Asterisk does all the logic and sends whatever line it wants down the Ethernet to your phone. Why would I ever need a voip phone with more than one line? I know there is a reason since they sell phones with many lines. I just don’t understand why. Pending my thick head finally understanding that, is there documentation on how to setup a phone with multiple lines/extensions?