There was a program where they would randomly assign TSA precheck to people (who aren't in the program). There were a number of reasons - trying to show people how "easy" it is (hoping that would get them to sign up); lightening the load at the regular TSA checkpoints; usually senior citizens who were considered "low risk".
Twice we flew and I got TSA precheck, but my husband didn't. It seemed silly for me to utilize it if I still needed to wait for him. So I would just go through the regular check point, but I wouldn't have to remove my shoes, belt, or jacket, while he did.
I've read that they've stopped this practice (random precheck assignments) because they have enough people who are participating to make the program worthwhile on it's own.