Why is the corpus luteum necessary?

Answer 1

The short answer is that the corpus luteum secretes progesterone, which enables gestation by preserving the cushy endometrial lining (thus giving the embryo a place to grow).

If there is no corpus luteum, there is no progesterone, and therefore the endometrial lining will slough itself off as menses.

Answer 2

After the egg is released from the follicle what remains behind in the ovary is called the corpus luteum.
It is just a necessary part of human biology