mgrand explained it better than I could. FG% doesn't tell the whole story, so there's more ADVANCED stats to accurately reflect a player's shooting.
Plus I already gave an example with something like rebounding. Traditional rebound stats just tell you how many rebounds a player grabs per game, but doesn't reflect the pace of their offense (more shots = more opportunities for rebounds), or FG% (lower % equals more missed shots and more rebound opportunities). So something like Rebounding % tells you the % of rebounds a player gets while he's on the floor.
Advanced stats are useful when traditional stats don't tell the whole story.
And again, no one is saying you have to subscribe to EVERY SINGLE advanced metric out there... but there are some basic ones that make a lot more sense than traditional ones.
And JRS, because what one person's eye test tells them might not be the same as what yours tells you. You might see a player and say "He's a great scorer, he shoots it well and can hit from anywhere on the floor" because you saw him one game hit a variety of jumpers or score from the post. When in reality, that's not really how he plays. That on average, he might be a worse shooter than you think or see. That he really isn't that efficient from a certain spot on the floor despite what your eyes tell you.
In simplest terms, because the eye test is subjective. I'm not going to swear up and down that "numbers never lie" and can't be misleading or misconstrued, because they can be... but what's the harm in having the data and numbers to backup what your eyes tell you? If everyone is so great at judging and evaluating, then the numbers should back it up, right?