The downloader asks for your account details, specifically your Steam account details because it needs them to download the game.

Where does it download the game from?

It downloads the game from official Steam servers. Steam has a system that uses storage servers that we call depots. These depots have all of the different games on them. They also have older builds of games archived on them, and that is where we get our games from! In order to download these old builds, you need one thing, and that is a valid license of Rainbow Six: Siege. That is why you need to supply your Steam account with the game on it to the downloader.

Is it safe to give the downloader my account details?

Yes, it is. The downloaders we use and provide to our members are fully open source and available on GitHub. If you do not trust our word, you are always welcome to go to the GitHub repository, review the code, and compile your own version of the downloader!