[iTunes] "Manually manage music and videos"

[Michael Prete]

Using iTunes v9.2, I notice that although I continually check the box  
on the Summary tab for my iPhone next to "Manually manage music and  
videos", the next time I open the app, the box is unchecked.

First, I would appreciate a clear explanation of the purpose and  
result of checking that option. Why would one select this option?What  
changes when this is selected?

Second, what might cause this option to deselect itself with no input  
from me?

Michael Prete
2.0 GHz G5 iMac, 17"
1.5 GB RAM, OS 10.5.8
iPhone 4, iOS4