here are just some of the reasons to use the sketch function:
--You can use metallic inked pens/pencils (something you can't do with printer) If not metallic, printer does just as well producing colors, etc
--You can use 12" wide media, which most printers (Unless you have a wide format printer) can't do--also you can use longer media in the Cameo as well.
--You can use a wet embossing pen to sketch onto stuff, then add embossing powder and heat with heat gun to get interesting raised effects/colors. Don't think a printer can do this.
See my experiences doing this here:
--Now I can add sketching with pencil! which can be erased later, unlike a printed image.
And here are some videos showing machine sketching on watercolor paper
And here is the Cameo sketching very detailed design of foxes (again, not my design), which I will work on in the next few days, really impressed with ability of Cameo t do this:
and here is a picture of the finished sketch: