Question: What role does keyboard resistance play when practicing at the piano?

For the same reason practice in tone-production without key or some substituted resistance is pedagogically unwise. The key resistance causes the muscular contraction-without the former, the latter is absent, and this brings us to an important point.
The distinction between the appearance of a movement and its mechanical nature. The characteristic

1. playing in the air or
2. on the surface of keys,

typical of pupils in whom finger-action has been unduly stressed is the result of separating the visual aspect of the movement from the mechanical. The only logical use of finger-action is the production of an appropriate tone, and this production depends entirely upon the speed of key-depression.
That is to say, upon appropriate overcoming of the key-resistance. I) We cannot judge the coordination from the appearance of a movement but solely from its mechanical nature.

These observations apply to any touch-form. The contraction of a muscle depends not upon assistance but resistance. Any external help given in a movement absolves the muscles from the necessity of doing that much work, and results in an incoordinated movement so far as its pianistic value is concerned.
All exercises in which theunuscular act ivity is accompanied by external assistance do not permit the proper coordination. Exercises for increasing the various stretches may help, if carefully done, by slight ly increasing the extensibility of the tissues; but they do not develop the muscular coordination which, later on, is used to at tain the stretch in actual playing.