You just use the program to make a Spartan. Any kind really. There's a lot of combinations too.
Although the program can mainly be used to create a dream spartan and reach the rank and credit goal, you can also post some Spartans created for fun. This program is also useful for use in Halo fanfics to create fan made Spartan protagonists.
You can also recreate characters from the Halo books too. We've seen actual depictions of Fred and Kelly from Halo Legend's "The Package", and now that we've got Vanity, we can recreate them. I'll show you what I mean in a moment.
But yeah, that's the point of the thread.So in a nutshell: Make a Spartan or recreate another Spartan that hasn't been depicted in the games yet for fun. If there's a certain Spartan that you've been willing to make but don't have enough credits in the game to complete him, then Vanity will help make him/her without all the hard work, also providing how to get there, meaning the rank and credits needed to make that Spartan.
Sorry for not being too specific.