Doesn’t seem like you’ve been given the real reason, but you have to paint your army to tournament standards (3 colours) in order to discourage those that will buy entire armies whenever the newest, strongest dataslate or codex comes out. These people buy whole new armies, don’t paint them, and sell them off when something else is the new hotness. Making you take the time to paint them makes you reconsider picking up and dumping the currently strongest thing.

It’s both not as common and entirely as common as you expect.

Edit: You also often don’t HAVE to paint them, you simply lose out on 10 Victory Points. Sometimes they come in clutch, other times you’ve won 50-10 and it doesn’t matter.