Why did Iraq invade Saudi Arabia?
WTF are you talking about?
How important is the Saudi Arabian army? and why?
Pretty important. The country is a monarchy where the only way to solidify power is a strong army. The government was formed from a strong conquering army and needs one today. It doesn't get many threats, except for terrorists like the late Osama bin Laden who wanted to kill the royal family. Also it fight terrorism throughout the country actively and consistently. There was a small 'war' a few years ago and may still be going on against border rebels on the Saudi-Yemeni border.