They synchronise the sheeps' cycles so that, with the injection, they are all tupped promptly and you can have a tight lambing.
I left it too late to do it this year, when my neighbour left me the info it said have a tup yon side of a stock fence for three weeks, then the sponges for two weeks, plus the jag. I didn't start in time, but I am going to look into it for next year cos I really need lambing within a specific fortnight.