There are a number of reasons: 1) Some apps have a limited life span - games, for example, or apps specific to an event (like the SIGGRAPH or GDC apps) 2) Som

There are a number of reasons: 1) Some apps have a limited life span - games, for example, or apps specific to an event (like the SIGGRAPH or GDC apps) 2) Some apps have bugs that you don't encounter until you use a particular function 3) Some apps stop being updated and you need more functionality than they offer. 4) You have a new need you didn't have before so you need a new app or apps to fulfill it