Why --force with mv?

If I try to rename a file with 'svn mv' and it has been modified then I get a
message like

svn: Use --force to override this restriction
svn: Move will not be attempted unless forced
svn: 'LDAPIf.h' has local modifications

What am I being protected from here? Is there some risk to the version tree or
am I being protected from doing something dumb?

-- 
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