Typeset labels change the nature f your files and your relationship to
them. Labeled files feel comfortable on a boardroom table; everyone
can identify them; you can easily see what they are from a distance
and in your briefcase; and when you open your file drawers, you get to
see what looks almost like a printed index of your files...
The main argument by David Allen is that in his GTD system, the brain need to feel relaxed at all times and that is an inherent advantage of having neat, clean, uniformed labels on your files. When the brain can relax about things other than what is the task at hand, the system is effective. A lot of what he teaches is all about repetition and relaxation, despite it being about actually getting things done.
The secondary argument for a labeler by DA is:
If you have to wait to do your filing or labeling as a batch job,
you'll most likely resist making files for single pieces of paper, and
it'll add the formality factor...
So you need something formal, fast, easy and (as always) fun. That's what a labeler can do for you.