The first thing to notice is that the scale has six marked edges. If you’re in the US, they will typically be multiples based upon an inch.
As you rotate the scale, you will see edges marked from 10 to 60 units per inch. That is to say, there is a side each for 1:10, 1:20, 1:30, 1:40, 1:50, and 1:60.
Note that there is also something called an architect’s scale, which is not base ten. It’s got less useful (except to architects?) dimensions, such as 1:3/4 or 1:3/32 or 1:3/8.
A diagram, map, or other drawing can be made to a standardized, consistent set of proportions with a tool such as this. An engineer’s scale can also be used to measure on a professionally produced map or diagram.