Why? The triangles are created by drawing the
diagonals from one
vertex to all the others.
Since there would be no diagonal drawn back to itself,
and the diagonals to each adjacent vertex would lie on top of the adjacent sides,
the number of diagonals from a single vertex is three less the the number of sides, or n-3.
The number of triangles is one more than that, so n-2.

This can be used as another way to calculate the sum of the
interior angles
of a polygon. The
interior angles of a triangle
always sum to 180°. The number of triangles is n-2 (above).
Therefor the interior angles of the polygon must be the sum of all the triangles' interior angles, or
180(n-2).

Irregular Polygon case

For
convex ,
irregular polygons,
dividing it into triangles can help if you trying to find its area. For example, in the figure on the right, it
may be possible to find the area of each triangle and then sum them.