Linked from
The 62 pages that link to Boundary layer, each with the reason it gives.
HydrodynamicsBroader topic: It explains how solid boundaries alter the surrounding liquid's motion.
Shear stressBroader topic: Strong velocity gradients there generate wall shear stress.
Kinematic viscosityBroader topic: Kinematic viscosity helps determine how thick this region becomes.
Transport phenomenaBroader topic: Boundary layers determine wall shear and surface transfer rates.
CurrentBroader topic: It explains how a current slows near walls, beds, and other surfaces.