I'm pleased that it seems understandable.
Templer has two modes of operation:
1. Files are processed/copied from beneath the input tree to the output tree.
2. Files are worked on "in-place".
In the latter case only files that need to be rebuilt are ever touched/modified/updated.
The same applies with the "input/ -> output/" mode of operation, but there is the caveat that I expect people run "make clean", or similar, every now and again. If you didn't, if you just re-ran it every now and again only the things that had changed, or were missing, would be updated.
(Assets are actually copied each time, if you're not running in-place, but we use tar to do it intelligently, so timestamps, etc wouldn't be changed unnecessarily.)