What type of basic drawing conventions are used and why there is need to be different types of drawings?

There are basically

three types of drawing conventions as:

Orthographic

Projection - A means of producing an accurate working drawing of a

3-dimensional object using (usually) three separate 2-dimensional

views (elevations) from three different viewpoints. The most

commonly used variation of Orthographic Projection are known as

Third Angle and First Angle.

Isometric Projection

- A means of producing an accurate drawing of a 3- dimensional

object using a combination of 30º lines for lines which are

horizontal in real life, and vertical lines for verticals.

British Standard

Dimensions - An internationally recognized set of rules for

applying dimensions to working drawings. Millimeters are always

used.