PHOTOGRAPHS FROM OUR OWN WORK
What this looks like in practice.
Water follows the available route
A valley carries water from adjoining roof slopes. Moss, leaves and twigs can obstruct that channel until water backs up and travels sideways. This can produce an indoor leak without a broken tile being the original cause.
An inspection matters
A blockage is not the only possible explanation for water ingress. The valley, nearby junctions and visible condition need assessing. Clearance may require safe access to debris concealed beneath tile edges.