It allows you to do dry hopping for a longer time. If you do one addition, common wisdom is that after 5-7 days you may get vegetal or grassy notes from the hops, in addition to the hop aroma you are looking for. If you do multiple additions, you can add pull out the first hop addition after 5 days, put in the second hop addition, pull it out after 5 days, etc.
FWIW, I usually do one dry hop addition, for around 5 days.