Dear student,
Tail of lipids in plasma membrane are hydrophobic in nature because of the following reasons.
1) It maintains the stability of cell membrane.
2) It acts as barrier to prevent the free movement of water soluble molecules from inside of the cell to outside and outside of the cell to inside.
3) The double bond present in the hydrocarbon chain provides kinks which makes it loosely packed and is responsible for the fluidity of the membrane. Tails move freely in the membrane.
Regards