Continuous addition of sugars in 'fed-batch' fermentation is done to obtain high yields of desired products. Fed-batch fermentation is a fermentation process where a substrate (such as sugar) is added intermittently or continuously to a bioreactor during the fermentation process. This allows the process to be maintained for a longer period, with a steady supply of nutrients to the microorganisms, leading to higher yield and productivity of the desired product. This method is often used in the industrial production of antibiotics, enzymes, and other products.

Thus, option A is the correct answer.