It is done to increase the probability of conception. You can read more about IVF and its need. This blog explains wonderfully what is IVF treatment and why is it required.
We need to understand that the success of any IVF programme the world-over is only 40–50%, no matter what anybody says. So, there is still a 50% chance that your IVF will fail for no reason. This is very important to remember as the cumulative embryo transfers- that is when you do successive embryo transfer with embryos that are frozen and check https://ifg-ivf.com/price/cost-of-ivf-programs.html , the cumulative pregnancy test positive rates can go upto 80%.
That said, there are three reasons why an IVF cycle can fail.
Problems with egg/ sperm or embryo
Problems with the uterus where the embryo is being transferred