R Yaakov Goldstein in his book The Laws & Customs of Mourning Vol. 2 writes (here online)
An Avel [who is the son of the deceased, whether the deceased is his
father or mother], is to lead the prayers as Chazan for every
weekday prayer, if he knows how to do so. [This applies even if
there are other potential Chazanim available. He is to push himself
to lead all the prayers, even if he does not have a good voice.]
Having the Avel [for a parent] lead the prayers is of greater benefit
[to the deceased] than the saying of Kaddish Yasom, as Kaddish Yasom
was mainly instituted for children [under Bar Mitzvah] who cannot yet
lead the prayers.
The reason why Davening is beneficial for the soul
is not because it contains a prayer or supplication of pardon on
behalf of the deceased, but simply because the son is performing a
great Mitzvah by Davening and sanctifying Hashem’s name in public, and
every Mitzvah a son does benefits his parent. It is understood from
here that the same applies towards any Mitzvah that a child does after
the passing of his parent, that it brings his parent atonement and
benefits his soul.
Furthermore, learning Torah and performing
charity and good deeds is of even greater benefit for the soul than
Kaddish Yasom and leading the prayers, as Kaddish Yasom was mainly
instituted for children [under Bar Mitzvah], and hence one must place
his emphasis mainly on these activities.
See there at length for details and sources.