When you start editing you'll rapidly see why.
The mixing process frequently involves you wanting to have access to several things at a time--for example, yesterday I was mixing a recording done by somebody else that had one master take of a female vocal plus two additional tracks with partial takes of the same vocal. I was using volume envelopes to fade between the three tracks to use the best portion at any given moment--and I had to be well zoomed in on each track to work accurately.
At the same time I needed a mixer window, a file window and sundry control buttons. To do this, I had the track waveform display on one monitor and the rest on my laptop's normal screen.
(and a small dog jumping into my lap trying to walk on the keyboard but that's another story....)
So, that's the long way of saying that multiple monitors are used simply to have multiple displays open at the same time--even if you're using just one piece of DAW software.