There are two reasons to wear them outside of situations like festival outifts. First to prevent a more serious problem if the girl has a wardrobe malfunction, which is what happened here. Second is if you're not wearing a bra, and you get cold, it hides the pokies. (Third reason is its more comfortable than a bra like Sylsai said.) And it's very obvious from the past year of her content she probably wears bras less often than your normal streamer.
It's always the simplest explanation that is true. Not these convoluted conspiracy theories some of you have. Poki has literally nothing to gain from doing this on purpose. She is way too rich and successful to ever do anything like an OnlyFans, and if she wanted to cause a controversy for whatever reason, there's so many other things she could do instead of anything related to her body. The average intelligence of this forum is so freaking low because no one here thinks with their brain.