why we reflux a reaction instead of direct heating?

Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in chemistry to supply energy to reactions over a long period of time.
If you merely heat the reaction mixture, you lose the volatile component which results in you having to stop heating when there is insufficient liquid volume.

Refluxing a reaction mixture is the best way of controlling the temperature with no sophisticated thermostat controllers needed. To carry out a reaction at any temperature below 90 deg you can simply choose an inert solvent with the right boiling point an stand the reaction vessel on a steam bath under a reflux condenser.
At higher temperatures use the same process but use electrical heating or an oil bath.