(19:9) A ritually pure man shall gather the ashes of the cow and place it outside the camp in a ritually pure place, and it will be for a safekeping for the congregation of the Children of Yisrael for sprinkling water, for purifying.

A third of the ashes of the red heifer that Moshe made were kept and were added to all the future red heifers. But why is this necessary?

The reason is because every mitzvah requires thought, speech and deed, as it says in Devarim (30,14) “with your mouth and with your heart to do it”. “With your mouth” refers to speech, “with your heart” refers to thought, and “to do it” refers to deed. 'Thought' is having in mind the reason and deep explanation (סוד) of the mitzvah, 'speech' is the blessing which is recited or the declaration of intent (לשם יחוד) that is said before doing the mitzvah, and 'deed' is the actual doing of the mitzvah.

But with the mitzvah of the red heifer it is not possible to do the 'thought' part, because the reason and deep explanation of this mitzvah were only revealed to Moshe. Therefore, it is necessary to add from the ashes of the red heifer that Moshe made so that all the other red heifers will also contain the 'thought' aspect.