PCR reaction cocktails!

[O.E. Barnett]

Can anyone help me? I'm trying to set up an experiment to
look at genetic diversity within a pop of Drosophila 
using PCR illustrated by RAPD analysis. Anyway, I've been 
advised to mix a single "reaction cocktail" rather than 
lots of seperate ones for each reaction tube.Why?- what
is the advantage of this?
 If anyone can help, I'll e-mail them a pint!
Thanks, Ollie.
              BGY6OEB at leeds.ac.Uk