I sketch when I try to explain something to myself. Drawing helps thinking. Those representations of an internal dialogue are abstract scribbles that only I can make sense of when I discover them again later on in my book.
I also sketch when I try to explain something to someone. Drawing helps talking. Those sketches are using a more universally intelligible sketching vocabulary, but have a relatively short life. Drawn on any piece of scrap paper I usually leave them behind or in the bin after a meeting.
I usually try and make pretty images for a report, or a presentation. Drawing has a greater impact than words. A good text can say so much more than long-winded text lines. A good sketch also has a longer lasting effect, as most of us remember better images than words.