It's hard to do with slate tiled roofs. The slates have to overlap to keep water out and the tiles aren't flexible. So when you get to a corner you basically start the layering process over again.

They sell corner pieces for slate, but they're pricey. So you typically see them do metal flashing in the corners, or some fancy tiling like in the post.

I'm guessing they just did the corners in this post, the rest of the roof looks older. Probably just replacing some bad flashing with something more permanent.

The only thing I can think of is that it would mitigate the risk of leaf litter corroding the flashing. It does add cost, so profit, so there is that.