COPY vs APPEND for imapsieve?

[MRob]

The examples on the imapsieve documentation page use COPY but not APPEND 
to watch messages coming into a folder. Are there not circumstances that 
a client would APPEND a message to a folder instead of COPY? Is APPEND 
only used by most clients for adding a sent message to the sent folder? 
I'm not familiar enough with IMAP clients to know if by leaving off 
APPEND I am safe or I am at risk to miss some actions I shouldn't.