Q.Why is a condenser used in distillation process?

Distillation separates (i) a volatile liquid from non-volatile impurities, or (ii) two or more liquids with a sufficient difference in boiling point, by boiling the liquid into vapour and then condensing that vapour back into pure liquid.

In simple distillation, used for a single liquid that boils without decomposing, the liquid sits in a round-bottom flask (heated by a water/oil bath or wire gauze) fitted with a thermometer (bulb just below the side arm); vapour passes through the side arm into a water-jacketed condenser, condenses, and trickles into a receiver. A mixture of acetone (b.p. $56\,^\circ C$) and water (b.p. $100\,^\circ C$) can be separated this way, since the two boiling points are far apart: acetone distils first, then water, with receivers swapped in between.

When two liquids' boiling points are too close for simple distillation to separate cleanly, fractional distillation is used: a fractionating column is inserted between the flask and the condenser, and as vapour rises through it, a repeated vaporisation-condensation exchange with the descending liquid progressively strips out the less volatile component, so the vapour reaching the condenser grows steadily richer in the more volatile liquid while the flask's remaining liquid grows richer in the less volatile one. Acetone/methyl alcohol and acetone/benzene are separated this way, and the same principle, at industrial scale, separates the fractions of crude oil in the petroleum industry.

Liquids with a very high boiling point, or that decompose on ordinary heating, are purified by distillation under reduced pressure: lowering the pressure above the liquid's surface (with a water or vacuum pump) lets it boil at a temperature well below its normal boiling point, avoiding decomposition. The soap industry uses this to separate glycerol from soap.