Why do need to stir the water continuously during heating?

by heating we increase the inter moleculer space of the solvent so the solute particles get in the increased space and by stirring also the same purpoose is served . in short it is done so as to dissolve the solute in the solvent

Heating will help solutes dissolve in many solvents. For example salt will dissolve in hot water better than cold water.

Many chemical reactions either require energy input to initiate a reaction, or they react better at higher energy levels.

Endothermic reactions require energy to proceed.

Even exothermic reactions may require activation energy to initiate the reactions.

Or... many reactions won't run efficiently or completely at cold temperatures.