Some women do not have regular cycles or maybe none at all in which case medication is needed to induce the ovary to respond to produce an egg.
If you are in an older age group or if you have a lower ovarian reserve, medication may be needed to produce a few more eggs to optimize the chances of conception. This is then referred to as ovarian stimulation or superovulation, at one extreme. Superovulation is mostly out of vogue now due to the risk of multiple pregnancy and its inherent risks but has some limited use with certain women. (See Mini IVF)