Application performance management (APM) isn't just for system administrators. It can and should be an integral part of the software development cycle. Some problems show up only after running for a long period of time, and APM is one of the best ways to make developers aware of them. Not every bug immediately produces incorrect results. Some of them make the application slow down over time. Some make it crash after a long period or when an unusual situation occurs. Code which works well with test data may not scale well as its database grows. Normal testing won't always catch these issues.
Network engineers and network managers tend to think of application performance only in terms of speed: their goal is to make the application as fast as possible or, at the very least, as fast as it needs to be. Stability and predictability, however, are two additional factors that contribute to performance. A stable application results in a consistent user experience, meaning that it performs equally well during peak hours and off-peak hours. As its user base grows, even if suddenly (e.g., after a viral marketing campaign), the compute demands will increase. Therefore, a high-performance application should also be designed with predictability in mind. In other words, it should be known in advance how an increased load under various conditions will affect the infrastructure so that the appropriate actions can be taken to avoid performance degradation.