The triangle shown can draw perfect free hand lines from point A to an exact point B.
It has been beaten against some hard surfaces and at times shaven a bit with a utility knife.
The free hand edge offers speed, consistency and accuracy.
By shaping the edge and by the line weights applied, the level of the free hand look is custom managed by the user.
The triangle is easily portable and when used with a T-square that has also been altered to look free hand, will speed up the sketching process by reducing the erasing of some lines.
These are handy tools for textures such as shingles that require repetitive lines.
The edges of an adjustable triangle treated this way would speed up free hand isometrics.
A wooden ruler altered as a free hand edge would be a nice portable tool but not offer the versatility of the triangle and T-square. Being softer however, it would probably generate more pronounced squiggles.
Tip:
A few of these free hand edges on hand, maneuvering the pencil and line weight would help not making these lines look totally alike.