To help form biologically important structures in cells.
Since most of all living organisms are made of water, and most nutrients and essential substances are polar, we need a stable substance that won't interact with them to help organize the distribution of such substances. This is where the Hydrophicity of Lipids come into action.
Cell membrane, cellular organelles, cellular vesicles are all made from a bilayer of lipids. And they remain intact of because of their hydrophobic nature. Otherwise, they'd be soluble in water and will be easily washed away, failing to do their job as a stable, intact structure.
A very good example is how the hydrophobic cell membrane would entrap and prevent any Glucose molecules from escaping outside the cell. Hence, preventing the loss of the main energy source for the cell.
Another good example is how the membrane of the Mitochondria traps in
And the list can go on.
I hope that answered your question.