Why are we drawing the isometric projections at 30 60 degree.?
Isometric projection is always done at an angle otherwise one of the faces would still be invisible. As to the reason for 30 or 60 deg, it is because in the olden days engineers were using set squares which had 30 and 60 degrees. (Set square is a right angled triangular piece with the other two angles being 30 and 60 degrees) Thus it was easy to draw anything at 30 or 60 degrees by keeping the set square on the T-square. You will understand this only if you use a drawing board for the drawings instead of using auto CAD/ Probably this practice has continued.
