Reason:
The price and demand relation helps in finding the position at which the demand of the product will be maximum which gives maximum profit at this point. But the marginal cost and marginal benefit relation gives a production point at which one extra unit produced will generate no value for the company. So this marginal cost and marginal revenue sets a limit for a maximum production at which the benefits generated are maximum. This means we have to stay maximum at the point at which the marginal revenue equals the marginal cost.