So I guess I should start this with a disclaimer that I genuinely respect authors who include OCs because I do think it takes a lot to create your own character in a fanfic that has their own unique traits. Using idols basically creates a foundation to a character that you then build around, which imo makes it a little easier to character build. This also, obviously, is not a personal attack on anyone. It's just something I've been wondering recently.
 

At the basic level there are two categories of fanfics: idol paired with idol and idol paired with original character (which also includes 'you' stories). 

I'll admit that I haven't read many OC stories until recently because I always preferred both reading and writing idol + idol stories. It's pretty common with OC stories to use a female idol's face rather than a model or actress—most likely because we universally agree that idols are beautiful and we like to look at them. Nearly every author well-known for idol + oc stories does this. What I find interesting, however, is when authors use both the face and name of a female idol as an original character. 

Is it a way to separate your story from crackships? To get more readers by having an OC? I'm not really sure.

This might be me thinking too much into things but I feel like turning an idol into an original character in a way strips that idol of their achievements, fandom, and essentially their identity. I also think that it says a lot that this isn't done with male idols (to my knowledge, or at least to a much lesser degree than with female idols). I can't help but see it as placing the importance of male idols over female idols. Essentially: male idols get to be viewed as people and female idols get to be viewed as interchangeable love interests—which stems from very deep-rooted patriarchal beliefs of the value and worth of men over women I will not be getting into here lol.  

For the level of fantasy and imagination employed in fanfiction, especially when so many are set in a non-idol world, most idols in stories naturally have a degree of separation from their real life counterpart. So that's probably why I don't understand the point of using an idol as an OC—to me it's like saying, this is not for your fandom, she is not yours anymore. Maybe that's extreme and not necessarily true, but it's the impression I get as a reader.