Currency collecting is much like coin collecting, but with added pitfalls. In coin collecting it is possible to damage a coin, but it is so much easier to damage currency.
Currency is valued by its scarcity and its condition. Among the things that devalue currency are damages corners, folds that cause creases, and holes, including pinpoint holes. Another problem is currency is occasionally written upon. Since wallets usually fold, damage by normal usage is common.
Finding crisp bills is difficult, and preserving them in crisp condition is important when they are part of a collection.