Organizations make a significant capital investment in hardware, and they develop the practice of updating them with a new version of the application and configuration. Over time, this leads to different servers running in varied configurations, and troubleshooting them becomes a very tedious task. Sometimes, you have to keep running unnecessary resources when they are not needed, as you are not sure which server to shut down.
The inability to replace servers makes it challenging to roll out and test any new updates in your server fleet. These problems can be solved by treating your server as a replaceable resource, which enables you to move more quickly to accommodate changes such as upgrading applications and underlying software.
That is why, while designing your application, always think of immutable infrastructure.