Standing water, a lawn that stays a sponge until May, beds washing out downhill — surface drainage clears it before it works its way to the foundation.
A corner of the yard that never dries out is annoying on its own — you can't use it, the lawn dies, mosquitoes move in. But in Seattle it's also the early version of a bigger problem: water pooling in the yard is water sitting against the ground that surrounds your foundation. Clear it at the surface and you often stop a basement issue before it starts. Ignore it and the same water eventually finds its way lower.
Surface drainage is the lighter, cheaper end of the trade, and for a lot of homes it's all that's needed.
A gravel-and-pipe run through the wet zone that collects standing water and carries it to a lower exit. The core tool for a chronically soggy patch. See French drain installation for how they're built.
Grated boxes set at low points that collect surface runoff and feed it into a piped drain line. The fix for a spot where water sheets across and puddles.
Long linear grates across driveways, patios, and walkways that intercept sheeting water before it reaches the garage or the house. Common on the flat concrete aprons of Eastside homes.
Often the cheapest and highest-impact fix of all. A Seattle roof sheds an enormous volume of water, and downspouts that dump it right at the foundation are a leading cause of wet basements. Piping that water underground and out to daylight or the storm system solves a surprising number of "basement" problems at the gutter.
As with any drainage, the exit governs the job. Sloped Seattle lots can often run to daylight by gravity; flat ones may need a dry well or a storm tie-in where the city permits it. A specialist maps the flow — where water comes from, where it collects, where it can be sent — before recommending which of the fixes above (usually a combination) your yard needs.