As a penetration tester, one of the important things to look for in web applications is how they store and transmit sensitive information. The application's owner could face a major security problem if data is transmitted in plaintext or stored that way.
If sensitive information, such as passwords or credit card data, is stored in a database in plaintext, an attacker who exploits a SQL injection vulnerability or gains access to the server by any other means will be able to read such information and profit from it directly.
Sometimes, developers implement their own obfuscation or encryption mechanisms thinking that only they know the algorithm, and that nobody else will be able to obtain the original information without a valid key. Even though this may prevent the occasional random attacker from picking that application...