The main reason some iOS apps say this is because of how sensitive some iOS apps are to low memory conditions. It's true that iOS will automatically terminate

The main reason some iOS apps say this is because of how sensitive some iOS apps are to low memory conditions. It's true that iOS will automatically terminate non-foreground applications to free RAM, however this process is not instantaneous. It's fast, but sometimes all it takes is a few milliseconds for a poorly written memory-hungry app to crash due to insufficient memory. Another scenario poorly written iOS apps come into is being too aggressive about memory allocation, which can trigger iOS to kill the app (which looks like a crash to the end user). Even well-written apps can run into this problem, especially on older devices (only the iPhone 4, iPhone 4S and iPad2 have 512MB RAM. Everything else has 256MB or less.)