Dreamcast games which were originally from Japan often have a directory (a folder) called omake on their GD-ROM's. If you open this folder with a computer, then you will find cool extras like wallpapers!
Dreamcast GD-ROM's include an audio track that warns people not to play the discs on ("stupid") stereos or hi-fi's that will play data tracks as audio tracks as the noise from playing them could in theory destroy the speakers if the volume was set too high.
Special Dreamcast games included a custom variant of this warning track. One of the coolest one is from Skies of Arcadia where the main characters, Aika, Fina, and Vyse, warn the listener instead of it being one of the standard warning messages.
As for X+Y temporarily increasing the brightness of the screen's images, my theory is that this goes back to the old days before digital capture devices and digital cameras were practical and the gaming press mainly used film cameras instead. In order to make it easier to take screenshots for magazine previews and reviews, the developers added the X+Y option to brighten the screen so that video game journalists could photograph the television screen for the screenshots. By temporarily brightening the screen, it would make the images appear brighter and clearer on the film and thus in print. It would also allow the developers to avoid having to ask the journalists to fiddle with the TV's brightness or contrast settings every time they wanted to take a screenshot with their film cameras. All that would be required is that the devs would have to tell the magazine writers about the secret combo to activate "screenshot mode."