The cleansing properties of a soap or detergent such as sodium dodecylbenzenesulfonate (shown here) require the molecule to have both a hydrophobic region (shown in red) and a hydrophilic region (shown in blue).
That phospholipids form a bilayer for a cell membrane via the hydrophobic effect is due to the phospholipid's amphiphilic character. The fatty acid hydrocarbon tail chains (shown in red) are hydrophobic, and the remainder of the molecule (the the two carboxylate esters, the phosphate ester, and the alkylammonium cation; shown in blue) is hydrophilic.