Why is it necessary to heat an esterification reaction?
Basically the use of heat is to liberate the water molecule from the mixture of acid and alcohol that is $\ce{OH-}$ ion from alcohol and $\ce{H+}$ from acid that results in formation of ester.Moreover it helps in evaporation of water since esterification reaction forms an equilibrium.The removal of water from mixture forces the reaction to move forward.It helps us to get a large amount of product.Alternative to heating some dehydrating agents like conc. $\ce{H2SO4}$ can also be added.