Question about .depends files

[walt]

Buildkernel failed this morning, which I eventually traced to a .depend
file in /usr/obj which was dated two days ago.  This got me wondering
how 'make' decides, in general, whether to update a .depend file or not.
Is it always a 'manual' process, i.e. some makefile higher up has to
do a 'make depend', or can it be done automatically by comparing
timestamps?
Come to think of it I've never understood whether .depends files are
mandatory or optional.  Some packages expects the user to type 
'make depend' and some don't.  What make the difference?

Thanks for any newbie clues you can give me.