Problem: when you sample a reference point for the Clone Stamp or the Healing Brush tool, the overlay reference is not visible inside the brush, making it harder for you to use the tool.
Problem: when you're painting using a regular hard brush, the lines are not smooth. Instead, the edges are jagged - as if you're painting with circles.
Problem: you're trying to work with some Photoshop tools that use a brush, but the cursor is actually showing the icon of the tool you're using instead of a circular brush.