I don't think it was about adding to the story. The actress actually was pregnant at the time the movie was filmed, so it isn't as though it was a plot device.
There are 8.2 billion people in the world. 8.19 billion of them have never heard of and don't give a fuck about Charlie Kirk. Get over it.

I thought it was a great twist to the story.
The detective was helping her cope and get through her husband's disappearance and they obviously developed a strong connectionand had a relationship (or even just sex) that led to a baby.
And then she FINALLY lets go of her husbandafter 7 yearsand the detective is all "Yee hah!!!" and on their first date, the husband comes back! lmaoso great! Then the detective is all torn between doing his job and wanting the girl for himself, so he takes out his resentment on the poor husband.
And then when the husband goes missing againhe's implicated in the disappearance somewhat by his partner because he was right there making out with the pregnant chick!
It added a LOT to the story.by giving hope to the pregnant chick and a relationship and way out of her own personal hell for 7 years

Not sure how it added to the story. Re-watching now so hopefully I will find out.

It was the police officer Detective that was in charge of her husbands case. They were having a relationship though not openly. It isn't confirmed he's the father, but it seems likely.