1) They exist for several reasons. DVDx is a hidden channel, hidden because it has no function on its own, its only used by others. The EULA is a hidden channel, because its only needed when you first start the Wii. And now WiiCrazy has created a program to make hidden channels for use with his Crazy Intro preloader app, because they are legal whereas ones with banners cannot be legally redistributed and its hard to make them legally.
2)Yes they have special uses. DVDx allows DVDs to be played, HideandSeek channels (WiiCrazy's hidden channels) allow legal shortcuts for use with Crazy Intro
3)Currently the only way to run them is Crazy Intro. But DVDx probably shouldnt be run (dont know what will happen), and EULA, again dont know what will happen if it is run, but it would probably be OK. he only ones you'd want to run are HideandSeek ones, which can be launched by Crazy Intro. Currently no other channel loaders have hidden channel compatibility.