This is done for several reasons.

1. Women who are undergoing procedures or treatment which will affect their fertility. Namely, the commonest scenarios are endometriosis surgery, chemotherapy treatment (especially for breast cancer), and long term disease modifying treatments for autoimmune diseases. There is usually a window of opportunity to freeze eggs/embryos prior to the treatment and preserve fertility for later.
2. Elective fertility preservation. As a woman ages, the egg numbers and egg quality drops. This process is much more pronounced usually after 35 years of age. For women who are in current relationships and are wanting to delay childbearing, or to have the
   opportunity to have a large extended family, there is the option to freeze embryos first so that they have the option to use them later in life.
3. For women with very low egg counts who would like the opportunity to have the option of having more than 1 child.