This looks redundant? To cease sounding, doesn't lifting the bow slightly over strings suffice? See 8 mins 11 s at https://www.coursera.org/learn/string-quartet/lecture/XuKBT/aizuri-demonstration-quartet-no-2-iv-entruckung.
It's just a little visual flavor to the performance. As you see the quartet does it in sync so it's meant to be part of the overall performance. You don't have to do it as a string player nor will most players do it for every song, but as a performer you want to add to your performance any way you can.
I'd equate this to a band's fanfare ending to a song where they try to do several flashy things before ending the song some are auditory (like quick extra solos and fills, ect) some visual (windmills, slides, stick flipping, jumps, ect).