More Comfortable: When you have a slim wallet, sitting down on hard surfaces isn't as uncomfortable as with a fat wallet. Your butt will be on more equal footing and you won't have to deal with a pressure point on one of your cheeks. A rubber band is one of the cheapest and most slimming wallet options. Sitting on a short, rubber-banded stack of cards will be much more comfortable than an inch thick wallet.
Easier To Find What You Need: If you keep just the bare essentials in your wallet, you won't be fumbling around trying to find your credit card or identification when you get to the cashier. This will help you save some time when you're in a hurry to get on your way. It's also so much less of a hassle when you only have what you need in your wallet.
A Slimmer Wallet Looks Better In Your Pants: Have you ever met a girl attracted to men with large wallets protruding from their back pockets? Probably not. The simple fact is that big wallets make you look a bit silly. Especially if your wallet is more than an inch thick and you wear well fitting pants. Just imagine it the other way around. Would you find it especially attractive if a girl wore some tight jeans and had a big fat wallet in her back pocket?
Less Clutter: Do you have all sorts of things in your wallet that you hardly ever use or even look at? If so, do a little cleaning and get rid of what you don't use. If you have a frequent eater card for a particular restaurant you go to every once in a while, keep the card in your car or workplace instead of carrying it around with you wherever you go. It doesn't make much sense to carry these around all the time when you'll be using them very infrequently. It's not uncommon for people to never fulfill the free sandwich, coffee, pizza etc. requirement to ever use the card in the first place, making it an even bigger waste to carry them around.
Less Wear On Your Clothing: This is another one related to looks. When you carry a big heavy wallet in your pants pocket, eventually it'll make a groove in your pants. This groove is not very easily fixed and could result in buying new pants. To avoid this costly solution, it's best to just use a slimmer wallet, to begin with. To further reduce the possibility of creating a groove in your back pocket, stick your wallet in your front pocket instead. This will also reduce the possibility of your wallet being stolen.