"A" reason is that a drill may not spin its bit nicely and freely once in a material. If the bit "catches" (does not spin nicely and freely), thee entire apparatus will then be the focus of the torque.
Whichever of the two, the bit, or the entire-(not itself confronting the material's resistance like the bit does)-apparatus will tend to take the action of the torque and the not-the-bit components face only the resistance to movement that air provides, in addition to any resistance provided by any hold one has on the apparatus. In short:
If the bit stops, the entire rest of it wants to begin moving, and more so as one tries harder to make the bit move.
Unless one makes it harder for the apparatus to move, the bit will not begin moving and one is not really achieving the assumed goal.
One's hold on the operating handle is part of that and any resistance pressing against the "shoe" (shown in fred_dot_u's picture), is helpful. However, the only direct action resisting the apparatus spinning so far is resistance through the operating handle and the torque and force one places there stands a very good chance of breaking the operating mechanism, or at least wearing it much faster.
Hence the handle shown. Gripping it only risks snapping it off, not ruining the expensive part of the mechanism. Force applied holding it in place actively resists the tendency of the apparatus to spin itself and not the drill bit. This allows one to force the bit to be the place of least resistance to movement without lessening the pressure "into" the bit's action (for instance, the force applied to the shoe). That means the bit can be kept from simply spinning without accomplishing much.
There would always be an ability to do that by lessening "inward" force, but the range of such available would much less and therefore the success much iffier, than by using the asked about handle. Using it allows a much greater operating range leading to the ability to have much more finesse in those situations.
Of courrse, most drilling done with it would not reach these points and one would only need use the handle now and then. Perhaps even ever depending on needs.
(By the way, perhaps see if it might simply be removable by design, or if a more careful removal might allow you to re-attach it when needed. Or if not "this-or-not-this-there-is-no-other", you might hack it off and use a power drill for the few times you would have made use of it. Also if value to another might be of interest someday, "in-tact" tickles more money out of people than "at-tacked"...)