Phospholipids are amphipathic in nature. ie., they form a structure with a polar head and a non-polar tail.
The polar head is hydrophillic while the non-polar tail is hydrophobic. This property makes the phospholipids, a major component of cell membranes as they form lipid bilayers. The lipid bilayer has its non-polar side of both layers facing inside and the polar side facing outwards. This helps in maintaining the integrity of the cell in a hydrophillic environment. This also helps in maintaining the composition of cell and segregation of molecules. The passage of unwanted things across the membrane is prevented and specific carriers are needed for transportation across the membrane as the outer parts of the membrane are polar while the interior is hydrophobic in nature.
There are other advantages too to the amphipathic nature of phospholipids. The property makes them good emusifiers that can bind and stabilize hydrophilic and hydrophobic molecules together in a solution. For eg., an oil in water emulsion may be stabilized by the phosholipid lecithin.