Why is it necessary to complete the identification of a bacterium based?

The trouble with identifying bacteria is that a lot of them look exactly the same. So, identifying by shape and such will only get you so far. However, two kinds of bacteria that look exactly the same under the microscope could have very different physiologists. One may be able to use a certain sugar as an energy source while the other one cannot, for example. Therefore, you can use morphology (physical characteristics) to narrow down the possibilities, but then you must complete the identification using the bacteria's physiology.