Why when games are announce they have like a demo in expos/comic cons way before they come out?

Theres a game called naruto shippuden ultimate ninja storm generations for the ps3 and xbox 360 the game was announce june 17th and now its july 1 and it already has game play and stuff but the thing is the game does not come out until next year so why do companies do this stuff... are they still gonna make changes for the game and stuff? also so far unsg looks just like uns 2 just added new characters.. they will add more stuff right? i mean like i said the actually game comes out next year