She was using to announce something, to call the other Trolls or to start 'Everybody' song...

Nice, but have you wondered why it was a bell you hit - not any other instrument? - In my opinion the answer is simple: because it was Loud, so everyone would hear it - & also easy to play, so she could play it even as a Baby in the header picture!  ^_^  ( Some other instrument might have been too complicated when she was as young as that - & Baby Poppy managed to play the cowbell without any problems! :) ... - & she was a Really Cute Baby!  ^_^  - Wasn't she? :) )

( Yes, her adorable babyhood picture just made My Maternal Instinct Work! - I'm Smiling! 😊 )