BeReal started as an app for unfiltered photos and gained traction mainly in 2022, so it's been around for a few years now, but your point about contests still holds up. The thing is, a lot of those giveaways are smaller than they seem: the creator puts up a $50 USD prize or some random product, but the actual reach is minimal because the app still doesn't have Instagram or TikTok's scale. So yeah, there are people who've won stuff, but the odds are pretty low compared to contests on bigger platforms.
What nobody's mentioned yet is the screenshot thing 📸 BeReal literally notifies you if someone takes a screenshot of your photo, so some of those contests ask you to do exactly that to participate. Depending on who's running the giveaway, you'd be sending your photo to accounts that might not be that verified, and even though it sounds paranoid, there's precedent for bots collecting images from these apps to train models. It's not like it's some crazy risk, just something worth thinking about.
If you really want to participate, check who's running the contest (verified creator, history of past giveaways) and don't give any extra info beyond what they ask for. But honestly, the chances of winning something actually useful are low, so it's not worth it if it makes you uncomfortable 😅