you will use them for a million things. If you don’t have precision squares then a set of 123 blocks will be your best friend. they will give you a reference for flatness and square, as well as all the uses Boss listed.
They can also act as a file guide when clamped to a blade.
I didnt think I would use them but I'm a Tool Hoe and bought a pair, think they were 12-15 bucks? In addition to the uses listed I've used them to check for squareness, square up the work table on my grinder, same on drill press (table to quill), check for flatness on blades and scales. Sure there more!