Here, the Ibur can happen for two reasons. The first is, that through the Ibur of the Nefesh of a Tzaddik into this person, [the Tzaddik] helps him rectify his Nefesh. And through his merits, he [the person] is able to rise in the Olam Haba to the high level of this Tzaddik. The reason is that this Tzaddik helps him and raises him so that he can add on mitzvos and holiness, and this is the reason he enters the person.
A second reason is to benefit the Tzaddik himself which enters into the person -- because when he raises the person up and helps him to add mitzvos and rectifications, the Tzaddik gets a part of the benefit from his activities. This is the secret of what the Rabbis would say, that "Great Tzaddikim, even in their death they merit to have children, etc." This is, that the person into which he enters [does mitzvot], and he becomes like a father to him, to help him [like a father helps a child], and in return, the Tzaddik merits, as we described.