When you work fast and loose, one thing that's easy to neglect is the explanation. I find that I have to remember to describe everything with text. I have these sketches sometimes where I'll just make a few boxes that point to each other, and I'll come back and think, what I was thinking? I'll have no idea.
A sketch is useless to you unless you can come back and recall the details of your idea. It's even more useless when it's time to come back and pass that idea to someone else. Sometimes a high level of abstraction can only go so far in helping to sell your idea without detail, because most likely your idea needs to be picked up, understood, and acted on by someone else. Words are important.
When I look at messy sketches with words all over them and arrows criss-crossing between boxes, I'm reminded of the algebra teacher that repeatedly said "SHOW YOUR WORK." Showing your work in math is important to showing that you understand and can trace back how you arrive at a solution. Using annotation and markup is as important in sketching. It explains what's going on, and helps you visualize a path of interaction, and go back and retrace the steps when you reach dead ends. I'll take ugly sketches that show your thinking over pretty sketches that do little more than put lipstick on the pig.