Actually, speed is as important as any other stat because if a Pokemon has strong moves and good special attack, but has bad defense and speed, it might get OHKO'd (one hit knock out) by an effective or super effective attack. It might close to death, or maybe your Pokemon has low hp and is your last Pokemon. It needs speed and good attack and defense to survive and that's why speed is important. I might have missed something but I this is all I know.