The advantage of transferring a blastocyst embryo is that studies have shown that the implantation rate is higher with these embryos. Additionally, at this stage fewer embryos need to be transferred to achieve a pregnancy. 
Though there are benefits of a blastocyst transfer, not all embryos are guaranteed to develop to this stage. Hence, to be considered for a blastocyst transfer we recommend multiple good-quality embryos on day 3 to increase the chances of embryos developing to this stage.