Here’s an overview of what Fink needs to do in order to install a new package. A bindist (binary distribution) refers to both the .deb packages available on your local system as well as remote packages (either from Fink or from unofficial bindists).
As explained in a previous post, the source code is only necessary to build .deb packages, which in turn are only necessary to install files on your system. Ultimately, only the files that the package installed under /sw are needed in order to run it. However, keeping the .deb files available helps with subsequent installations and updates.