some microbiologists recommend inoculating a pair of OF basal media (without carbohydrate) along with the carbohydrate media. Why do you think this is done

This is to ensure accurate recording and interpretations of the observed results without Carbohydrate.

To ensure that the observed results was due to the expected investigating factor, and not due to any external factor.

Explanation:

The basal media without carbohydrate is the negative control setup. since there is no carbohydrate their will be no bacterial growth. Therefore the activities of the bacteria can be fully observed. However the basal media with carbohydrate have full growth since CHO is available as nutrient;, asexual reproduction will occur and growth takes palce. Thus the control (without CHO) and the actual experiment can be compared for accuracy. Therefore the basal media without CHO. acted as active negative control for results analysis and interpretation.

In addition the two growth media should be inoculated. Thus any observable changes can be attributed to bacterial growth, and not due to any external factors. This justify reasons for the answers above.

As a control, which, in this case, is a negative control. Since there is no carbohydrate added there will be no bacterial growth. In your tubes which do have some carbohydrate there will be bacterial growth. Since you used a control you can compare any changes in the tubes in which you have growth to the tubes without growth, which would represent the original state of the media. This makes any results easier to notice and interpret rather than having to remember what the tubes used to look like.

It also ensures that any change in the media is due to the growth of the bacteria and not some external factor. Since both sets of tubes were inoculated but only one set shows some change you can be sure that that change is due to the growth of the bacteria.