what the advantage of a 40x powered lens when viewing a specimen?

Answer 1

Optimal magnification depends upon application.
10x may be optimal for viewing a coin or flower.
40x may be better for the anatomy of an insect's head.
100x may be better for xylem and phloem in a plant stem.
400x may be better for blood cells.
1000x may be better for bacteria.
ETC...