Really boring! This is what has replaced lead for lining roof valleys (the join where two roofs meet at right angles). The wibbly bits down each side are for the tiles to sit on: the rough strips are to take the mortar used to point the join between the tiles and the lining.
Gosh that's boring. Sorry. But it should stop our nasty leaks (caused by fatigued lead and odd goings on in the tiles).