The wall tool is useful for adding walls inside (or outside) dungeon areas.
Erase walls by drawing over them with another tool - the rect tool is good for this. If the wall is inside a dungeon, you’ll need to be in draw mode. Use erase mode if the wall is outside the dungeon.
This tool is also useful as a generic line drawing tool. If you create a new layer then turn off hatching and shading, you can use the wall tool with snapping off to draw on the map. Arrows, lines, and other annotations are good candidates for this.