Since 1996, the Tokyo Game Show has given developers yet another reason to amass in a convention center to show off their wares. Until 2002 it was held twice a year, something that changed when it was decided to cut one, leaving only the the autumn show in September, remaining.
But that's the boring part. What's interesting is what actually happens once the show floor opens up. As it takes place somewhat late in the year compared to GDC and E3, most developers hold off from making giant announcements for the current year, but that doesn't stop them from revealing big titles before and during the event.
Hardware, like the Wii's controller and the DualShock 3, were first shown off at TGS, and every year new games are revealed with awesome trailers and announcements. Metal Gear Solid 4's first trailer debuted there (to incredible acclaim), as did the first details on Kingdom Hearts Birth By Sleep, Kingdom Hearts 358/2 Days, and Kingdom Hearts coded. In fact, it’s because of this that fans of Square-Enix franchises continue to watch the show--the developer often takes out the big guns once the doors open.
Generally, western developers don't reveal too much at the show, but it's not uncommon to get a new trailer or two from Microsoft or EA. Some publishers announce new stuff at TGS to avoid the hustle and bustle of E3 in favor of owning the word-of-mouth at Tokyo Game Show, a tactic which has proven successful in the past. It also means we get more cool gaming announcements, so, yay!
This year there were rumors of a PlayStation 3 redesign (which turned out to be true), more news on Wii U titles, and potentially new footage of The Last Guardian. It takes place this week, so expect to find out about this, and more, in the coming days.