DESTDIR?

["John G. Heim" <jheim@math.wisc.edu>]

I am working my way through the Debian New Maintainer's Guide at
http://www.debian.org/doc/manuals/maint-guide/ch-modify.en.html.

The program I'm working on uses a configure script to build a Makefile. So I 
need to change it to install the binary into /usr/bin instead of 
/usr/local/bin. But the new maintainer's guied says to put $(DESTDIR) in 
front of the directory names. Like:

      BIN     = $(DESTDIR)/usr/bin

But the Makefile already uses that variable name. What is the point of using 
$(DESTDIR) in maintaining the Makefile?