Your own replacements will cost a little more than the fat lambs you produce, which is usually substantially less than bought in replacements. You could argue that the maternal wether lambs are likely to make less than the terminal lambs that might otherwise be sold from those ewes, which is another cost to be carried by the system.
However, it allows you to breed replacements from known ewes, and with known disease status. There are a whole host of devastating diseases that can be bought in with incoming livestock, Mv and OPA being just two that are becoming more common, neither of which are given any consideration at most breeding ewe sales.
You need a better maternal breedYou need a better terminal......
But I forgot, you like proper sheep