Here's why:

* Mutations are the ultimate source of genetic variation: They introduce changes in the DNA sequence, which can lead to the formation of new alleles.

* Other processes shuffle existing alleles: While processes like gene flow (migration) and recombination can introduce new alleles to a population, they rely on pre-existing variation.

* Mutations are random: They occur spontaneously and can happen in any part of the genome. This constant introduction of new variations is essential for evolution.

In summary: Mutations are the primary driver of new alleles, providing the raw material for natural selection to act upon and ultimately leading to the diversity of life we see today.