“You can't do sketches enough. Sketch everything and keep your curiosity fresh.”—John Singer Sargent
Although this advice comes from an artist, writing is painting with words. Sketches are small pieces—writing practice. Writing sketches is a time to play with ideas and characters and description. If it’s outside the boundaries of a WIP, there is less risk. Write about people you see during the day. Don’t be hampered by lack of facts. Make stuff up. Draw assumptions and weave a story. You’re not writing about the person standing in front of you in line at Walmart. You’re merely inspired by them to write about a fictional character with only a surface resemblance (and even that is subject to change once you start writing).
Sketch characters, settings, ideas, descriptions, bits of dialogue, whatever captures your fancy. Think of it as a scrapbook you can draw ideas from.