I assume the line tool is usually used for spells that cast in a line.
However, It would be nice if it could be used for thin lines too, to enable drawing (e.g. you could draw an arrow on the map using individual lines, or to draw a proposed path across a map for overland travel
It could be another icon on the customization bar alongside colour.
For now I am working around it by creating lots of small circles, which have customizable size not locked to the grid size