Free Robux

Free Robux is a way of preying on small children who want their rainbow Adidas hoodies from the Roblox Store. There are hundreds of these scam sites polluting the Internet, and they are all fundamentally the same. You visit the site. It looks legit from a 10-year-old's idea of what legit looks like. The site asks for your Roblox username. It usually stresses around the text field that it definitely does not need your Roblox password, knowing full well that Roblox accounts are one of the most worthless things that can be acquired.
Then it asks you to select the amount of Robux you want to generate. Honestly, this shouldn't even be a step, because every 10-year-old will pick, without fail, the maximum Robux possible. Then it asks you to "verify that you are a human" because a 10-year-old would definitely make a bot that gets them free Robux. I'm sure it happens all the time. The way you "verify that you're a human" is by downloading all these ridiculous, obscure programs and completing a simple task inside it.
That is how the free Robux site takes advantage of children. Sketchy programs (stupid VPNs, low-effort game clones, and other crap) pay the site to advertise for them by adding them as an "offer". Stupid kids download these programs without a moment's hesitation, which makes the advertiser money. Now both the free Robux site and the sketchy program have made money, and everyone is happy except the 10-year-olds who don't get their free Robux because free Robux isn't real. Rinse and repeat.
Most free robux sites have trackers to see who is on them. They can get your IP Address, and if you run into a darknet site, they might be able to even track down your location.
Roblox doesn't give a crap about these sites because if they did, they would protect their young, fragile-minded playerbase from these scams by having the sites shut down. Technically, free Robux sites are illegal because they take advantage of the... uh... dimmer internet users by making them churn out fat wads for them in exchange for a fraudulent reward. 
In conclusion, free Robux sites are to be avoided at all costs.