In SR3, they actually had rules for using knowledge skills to supplement active skills (using psychology to gain an edge in negotiation test, etc.).
In SR4, they are only indirectly helpful. A knowledge skill of ork culture might let you fit in better in the Ork Underground, a knowledge of Italian cuisine might help you fit in when you dine with the don, etc. They can either give you a bonus (or avoid a penalty) for social skill checks, or give you helpful information from the GM (your gang identification skill lets you notice that the stuffer shack is tagged with graffiti marking it as being protected by a local gang).