Me got there before me re the AI, and explained it better anyway. Bearing in mind that you can potentially use the same tup on many more ewes, not really any more expensive, and if you have spent a lot of money on a tup, which dad had, you want to get your money's worth, I guess. Not for me, though, except perhaps saving a breed/bloodlines.
Re the above link, absolutely true, year on year, it's the Blackie Money-Go-Round. If you offset the tups a small group of farms bought off each other, would be interesting to work out what they ended up having laid out. Works as a very successful marketing strategy though, because there is no doubt that being seen to spend makes your tups just under the best 2 or 3 more sought after.