Why did she throw this tap valve?

It is not explicitly said in the movies, but judging from the situation and her body language, it looks like she is trying to distract the masked killer.

More specifically, the point seems to be to make him think she went the opposite way. Jess is definitely not in her right mind for most of the movie, so it is not unbelievable that she would do that right before loudly running the other way, even though it is very counterproductive.

This is supported by the fact that later in the movie,

when she is the masked killer, she notices the throw, but doesn't think twice before running the other way to ambush herself, because obviously she knows it's a trick.

Furthermore, in a more meta sense, I feel like this scene was added to tell us, the viewers, the exact moment Jess

truly accepted the fact that she was in a loop. Yes, she obviously noticed it before, but it was the first time she actually used her knowledge of a previous iteration to her advantage (knowing where confused Jess would run and ambushing her). Before that, she was mostly confused and trying to break the loop, or straight up denying it was one. When the throw happened, masked Jess instantly decided to run the other way because she remembered where she had ran and decided to take advantage of the loop instead of falling victim to it like pretty much every other time since the beginning of the movie.