The Adult's Wallet is a recurring item in the Legend of Zelda series. It is a big wallet that allows Link to carry 200 Rupees, which is 101 Rupees more than he could carry before. There is an even bigger wallet that can be obtained, though, known as the Giant's Wallet.
The Adult's Wallet is obtained by collecting 10 Gold Skulltula Tokens and seeing the first freed brother of the House of Skulltula. He gives you the Adult's Wallet without having to pay anything as a reward for freeing him. This wallet is needed for a couple of small things, one of them being for purchasing the last Magic Bean that the Bean Seller sells Link. The Adult's Wallet is very practical in many circumstances even when it is not needed.
The Adult's Wallet (now known as the Adult Wallet) is obtained by either depositing 200 Rupees into the bank, or by beating the Oceanside Spider House on the First Day. Again, the wallet is needed for a couple things, and is practical always.