Home computer use: downloaded installation files

Self-tutoring about home computer use: the tutor mentions an issue that might come up sometimes.

The other day, one of our home computers was complaining that it was nearly out of memory. Looking through it, one couldn’t help but notice the Downloads folder had swollen to many gigabytes.

The question arises: can one safely delete downloaded installation files from the Downloads folder once they’ve been run, and their corresponding program successfully installed?

I turned to the Internet and was happy to see Leo Notenboom, of Ask Leo!, discussing the question in this video.

Leo gives some interesting counsel therein. I got from it that, having successfully installed a program, one should perhaps preserve its downloaded installation files on a separate drive, should a re-install ever be necessary.