The ruler tool is like an axis. If you click on an object near the ruler it will show distances from the ruler in two directions. Like an axis the ruler has a positive and negative side.
To place a ruler, press the R key and click anywhere on the workplane or select the ruler icon. With the ruler placed, click an object and you’ll see all of its dimensions displayed, as well as its relative position to the ruler.
You’ll find an “x”, where the two axis meet on the ruler, to remove it from the workplane, as well as an option to measure relative distance from the midpoint of any selected object, instead of its endpoint.
In this mode, the ruler shows the dimensions from the endpoints of the object.
In this mode, the ruler shows the dimensions from the midpoint of the object.
The ruler tool can be handy for dimensions when using one object, but how often do you do a 3D design with only one object, probably never. So we need multiple objects And ruler is really helpful to know the distances among them
The ruler tool can also be helpful in making holes in objects. If you line up the tool with two edges of a shape, you can see the distance of the holes from those edges.