Question

Why it is necessary to use isometric scale for drawing isometric projection?

Answer 1

In isometric the three angles between the projection of the axes are equal, so the degree of foreshortening along each of the axes is the same. Isometric means "equal measure". This means that only one set of scales is needed to draw an isometric projection of an object.

Isometric projection are always drawn at an angle of 30 or 60 deg otherwise one of the faces would be invisible.

Answer 2

in isometric the three angles between the projection of the axes are equal, so the degree of foreshortening along each of the axes is the same. Isometric means "equal measure". This means that only one set of scales is needed to draw an isometric projection of an object.