How soakers work
Each soaker sits under a tile or slate and turns up against an adjoining wall or chimney, with the next course overlapping it — together they form a continuous, flexible seal along the junction.
Why they're used instead of a single sheet
Individual soakers allow the junction to follow the exact profile of the roof covering, which a single continuous flashing piece can't do as reliably on a tiled or slated roof.
Common problems
Soakers can corrode, or work loose if the tiles above them are disturbed, and because they're numerous and hidden under the roof covering, a failed soaker isn't always obvious until it leaks.