Everyone's reasons will be different, but their goal is the same. Women who freeze their eggs are looking to maximize their chance of having a healthy baby when they are ready.
For some women, age is a factor. Once a woman reaches 28, her chance of conception declines every year. As the chance of conception declines the likelihood of her eggs having chromosomal abnormalities increases, as does her chance for miscarriage. Egg Freezing allows a woman to protect the current health of her eggs which increases her chances of successfully conceiving through IVF if she should find herself struggling to conceive later on.
For other women, social reasons like focusing on school or a career might be a factor as shes does not have the time to meet or find the right life partner.
Lastly, medical concerns might be a factor. Some forms of early menopause, such as premature ovarian failure, has the potential for a woman to have significantly fewer healthy eggs than other women of the same age group. Certain severities of endometriosis require surgery to remove all or part of a woman's reproductive organs, possibly reducing the likelihood of conceiving naturally or traditionally. Cancer and other radiation or high-drug treatments can also cause concern for a woman's reproductive health. Freezing eggs before these procedures can ensure the possibility of having a biological child, even if you require a gestational surrogate.