I have a hard time grasping the point of hidden tracks or bonus tracks on albums, or tracks only on versions of the album made for specific regions. Especially since they don't seem to be any different from other tracks on the album, in many cases.
I'm only going to refer to bonus tracks in this:
In some cases, it's a way to sell an album to a certain region. For instance, the Japanese version of Metallica's And Justice for All features an extra track--a cover of the Diamond Head song, The Prince. Supposedly, the reason they did this was to encourage Japanese fans to pick up the official Japanese release of the album instead of importing the North American or European version, as apparently for a time, it was cheaper to import than to pay for the official version--at least in Japan.
In other cases, there's legal precedent. Megadeth's debut, Killing is my Business, featured 8 tracks originally, but later versions had only 7. They covered These Boots, and because the original writers were apparently greedy bastards, Dave Mustaine simply yanked the song from later releases. It became a kind of a bonus track for anyone with the older version of the album. Though this is not technically a bonus track, but illustrates why changes may occur sometimes.
The laziest reason to include bonus tracks is to encourage people to purchase re-releases of albums. The remastered re-release of Killing is my Business featured several bonus tracks, with These Boots as one of them--but they were forced to censor the parts of the song Dave rewrote, so it has fucking obnoxious loud beeps in it. And in that case, I can't help but wonder why'd they'd even fucking bother. No one wants to listen to that kind of shit.