Adrian runs FAST. He runs FASTER than Jacob <----- is

A helping verb is always accompanied by a main verb. The purpose of the helping verb do or does is to emphasize a point. Ex: I do need to take some medicine to put off my cough. Here you can see you hve two verbs, do, and need. Need alone may be enough to convey your thought but by reinforcing with a helping verb do, the point is further stressed.

In this given case does is a mere synonym or alternative for the verb runs; it is not used to add strength and hence not a helping verb

Now in this case, what are we comparing? Are you comparing Adrian with John or Adrian’s action of running with John’s action of running? Obviously, we are comparing the action of each other’s running. Therefore, an action verb such does which is a replacement for the main verb runs is required. However, some try to justify the first sentence saying that the second verb is elliptical and understood. That might however led to errors.

Look at the following sentences and derive which is better.

1. Nobody understands GMAT better than nobody understands Tom
2. Nobody understands GMAT better than Tom does.

The fist sentence may mean that nobody understands GMAT better than Tom does or nobody understands GMAT as well as he or she understands TOM. You can see the ambiguity

Here. It will be more straight and clear to say that nobody understands GMAT better than Tom does All this is in my view and I am liable to be rebutted