There are loads of web apps for sketching, but most output imagery that’s a low-res mess, meaning you can’t use it for anything useful. By contrast, Sketch is vector-based, so you could cover the side of your house in your masterpiece, if you could find a big enough printer.
If you’re an Illustrator whiz, you can draw Bézier curves with a pen tool. If not, there are pencils and brushes for freehand scribbling. You can even import photos to trace over, if the thought of starting with a blank canvas is terrifying.