"I'm not sure I see how or why it was ever necessary (or beneficial) for a 13ish-year-old boy to be groomed to perform sexual favors for a 30ish-year-old man who had a great deal of psychological influence "
Logical fallacy: argument from ignorance; What you understand or do not understand is irrelevant to discussion. .
As has been explained more than once. Ancient Greek morality was very different from our own in may ways. Homosexual love was seen as the ideal. The relationship between an adolescent boy and older man was seen as a great good. The man was the boys mentor. Yes it was a sexual relationship. That was simply seen as a natural part of the relationship, which usually lasted several years.
I recommend a read of the quoted passage below
Pederasty is an ancient Greek form of interaction in which members of the same sex would partake in the pleasures of an intellectual and/or sexual relationship as part of a socially acceptable ancient custom (Hubbard: 4-7). The question of whether the ideal pederastic relationship was the most common form of pederasty in Greece, or whether the reality of ancient same-sex desire involved relationships between males of the same age, is one that has been contested between scholars for many years.
The ideal pederastic relationship in ancient Greece involved an erastes (an older male, usually in his mid- to late-20s) and an eromenos (a younger male who has passed puberty, usually no older than 18) (Dover, I.4.: 16). This age difference between the erastes and the eromenos was of the utmost importance to the scheme of the ideal pederastic relationship. The
power dynamics involved in such a relationship, with the erastes always in control, ensured that the erastes kept his dignity as a fully-functioning member of Greek society, while the eromenos grew up under the tutelage of such a man and as such could become a great citizen when he reached adulthood. Both people in an ideal pederastic relationship would have practiced great sophrosyne, or taking no indulgence to excess (Dover, II.C.5.: 97). The erastes shows restraint in his “pursuit” rather than his “capture” of the young boy, and the eromenos would similarly show restraint by not immediately giving into the older man’s sexual desires.
Ideal pederastic couples were ones whose relationship directly benefitted their Greek society.
Examining Greek Pederastic Relationships