In Ridley Scott's film there's a point where Ripley (Sigourney Weaver) has the opportunity to escape. All her crewmates are dead, having encountered the alien and met a grisly end. There's no fighting the alien, it has blood so corrosive that if you did manage to wound it you would die because the ship's hull would be breached and all your air would disappear. So, all Ripley can do is get into the escape pod and get the hell out of there.
Because the film would just end there and then in a most unsatisfactory manner. There would be no resolution. And resolution is very important for any film or novel.
Ripley decides to rescue the cat and give the alien a chance to creep on board her escape craft so that we can have a proper and fitting climax to the film.
I remember shouting at the screen 'Leave the ****** cat and get the hell out.' The cinema was absolutely packed and there was a tremendous groundswell of agreement with my outburst. Although we all wanted Sigourney to be safe, we also respected and fully understood her decision to rescue the cat. She wasn't the kind of girl to abandon it to the dreadful clutches of the alien. We would all most certainly have left the cat and saved our skins, but the whole cinema audience knew she was made of much stronger stuff than we were.
It worked. Her actions, however we might disagree with them, were in character. So we got our ending and our resolution without it seeming contrived.
If you ask me, the big contrivance was the super corrosive blood. But I'm a chemist and I would be sceptical about something like that.
In the Due Diligence, Jenny is in terrible danger but hasn't got the option of running away from it. She has her own version of Ripley's Cat.