Linked from
The 25 pages that link to Incompressible flow, each with the reason it gives.
StreamlineRelated: In this common setting, streamline spacing helps indicate changes in flow speed.
StreamliningRelated: Many ordinary vehicle and swimming flows are analyzed under this approximation.
Hele-Shaw cellRelated: This condition helps make the depth-averaged velocity field divergence-free.
Low Reynolds number flowRelated: Many canonical low-Reynolds-number models assume incompressibility.