Hiya Oli,
The main reason for giving out a separate file per user is simply to make it easier for you. Having a single file with 20 activations would cause severe problems if even one of the users had issues.
Say that user No. 8 had problems and activated twice that would mean that someone would be without a license. The quickest solution to this problem would probably be to reset the entire license.
Having one license per users is a much more forgiving way of completing installing.
If the licenses you've purchased are renewal licenses they require a previous license installed, otherwise they shouldn't.
Renewal licenses function as time-adders, ie they need something to add time to.
