Where flashing is needed
Any junction on a roof — a chimney, a wall abutment, a dormer, a valley — needs flashing to bridge the gap and stop water tracking in at that joint.
Why lead specifically
Lead is still the standard material for roof flashing because it's malleable enough to dress tightly into place around irregular junctions while remaining durable for decades.
Getting a clean installation
Properly installed flashing is dressed, bedded and secured so it stays watertight through years of weather movement and thermal expansion, not just fitted to look right on the day.