What difference does rooting hormone make when propagating plants from cuttings? I have have been successful with and without it, but I would like to know for example, if it affects the speed or health of the growth.
There is a very full description of the various types of hormone rooting compound here.
As already pointed out, it is supposed to stimulate root formation; I have tried using it on cuttings a couple of times, but haven't fond it effective; in fact, if anything, some of the cuttings would probably have done better without it.
According to the New Encyclopedia of Gardening Techniques (Royal Horticultural Society):
. . . . If the stem cutting is propagated from a healthy plant and at the correct season, then the use of such hormones is usually of no advantage whatsoever.
The concentration of hormone applied to induce root formation is not the best concentration to cause root development.
By applying the hormone the roots are induced to form, but if they emerge and come into contact with the hormone.... this may cause the roots to die off.
It is meant to stimulate root growth - in particular to stimulate the creation of roots.
I've only tried it on peach cuttings so far, and haven't don't a comparison with/without. These had mixed results but for other reasons - first time it was the wrong time of year, and the second time most of the cuttings were too immature.