Many iOS apps violate the use of memory. This and other issues leave a lot of iOS memory unusable. Purging memory is the only way to clean main memory. There are also many poorly written apps that do not assign and/or use memory correctly. Many also make invalid calls, use arrays incorrectly, and/or violate program boundaries. Improper use of memory, however, is one of the top reasons apps crash. This app does a nice job of dealing with junk memory.
Marcus055 about Smart Memory Pro - Check RAM & System Checker