why is it a good idea for a bacterial cell to be abel to use glucose first as an energy source then switch to lactose

Due to less steps and requires less energy.

Explanation:

The bacterial cell is able to use glucose first as an energy source then switch to lactose because glucose requires less steps and less amount of energy for the break down as compared to lactose. If lactose is the only sugar available to the bacterial cells, then bacterial cells will use it as energy source for the production of energy. In order to use lactose, the bacteria must express the lac operon genes, which encode the main enzymes for lactose uptake and metabolism.