Besides wearing a uniform on the pitch (jerseys change by what colors the teams are wearing), a badge (HS league, USSF or club), a data wallet (game card, pen or pencil and yellow and red cards), a whistle is definitely needed in any game. The less I blow the whistle the better, but it's the only equipment to signal the course of the game for all to hear. When I first began refereeing, I was told that I had a "soft whistle," and needed to blow it louder (especially on windy days). I think with the plethora of whistles that I take with me, I'm there now in the "blowing-the-whistle-loudly" department. It's really our voice on the pitch.
My photo below comprises of the whistles I've used over the years, but my favorite is not in the photo (it's a Fox40 and fits on two fingers).
BSoleille! The bright side of toot toot, or now, TOOT TOOT!