[pluma]

I've heard several explanations.

The negative explanation is that it degrades the recipient (i.e. women), adding to the sadist/misogynist aspect of heterosexual intercourse.

The pragmatic explanation is that it provides a visual cue for the viewer (particularly those who are more visually oriented) indicating the climax of the sexual act. If the orgasm were internal, it'd be hard to miss (though there's an entire niche for that, of course). As to why it's the face in particular: it's the easiest way to directly show the recipient's reaction at the same time, especially on low-resolution or small-scale displays (plus on those devices a close-up tends to be easier to recognize).

The psychological explanation has something to do with there being a long-standing trope of portraying cannibalism as the ultimate infatuation (e.g. the communion of the Holy Spirit in Christianity representing "becoming one with Jesus" by consuming his flesh and blood) and ejaculate representing the "essence" of the male orgasm (biologically speaking the entire "point" of sex is to put one person's sperm in the other person's body).

It may seem odd that there are people who are into the ejaculate itself but it's far more straightforward than a lot of other things some people enjoy (including pretty much everything else that can come out of a person's body, apparently non-sexual body parts and things that aren't even related to persons, or even BDSM which is so much about psychology it can get away without any actual intercourse taking place).