It's weirdly addictive. It feels like every time I put FIFA in my Xbox I'm dropping £3 on imaginary football cards..
And yes, people can buy loads of great players, but it's more complex than that. Every player is different, you could have three Lionel Messi and they'll all be different, different preferred position, different preferred formation. The trick is to have a team with good "Chemistry" which is the measure of how well your players get on. Players with the same nationality have good chemistry, players who play for the same club have good chemistry and then players who have the same preferred formation as your team and so on. It's a fairly complicated system and in no way can you just win by buying the best players (although that will help).
EA are really smart with it too, I wasn't going to get FIFA this year, but they emailed me today and said I can start my FIFA 13 team on their website before even having the game. They give you a load of player packs to start out (which includes some decent players, surprisingly enough..) then when you've spent a little bit on the website it says if you pre-order the game they'll give you a ton of extra packs when you start playing the game.
They're like shady drug dealers getting you hooked on crack.