Your distribution’s package manager probably uses GPG signature checking to provide an extremely strong guarantee that the software packages you download have not been maliciously modified by a man in the middle (MITM) attacker when traveling over the Internet from your distribution to you. Smaller distros might have no such infrastructure in place (these distros are not safe to use), but for most major distros, a MITM attack between your distribution and your computer would be very difficult to pull off once your distribution has been installed. (Installing a distribution for the first time is another matter.)
But what guarantee is there that no MITM attacker compromised the tarballs when they were downloaded from upstream by a distro package maintainer? If you think distro package maintainers bother with silly things like GPG signature checking when downloading tarballs, then I regret to inform you that Santa is not real, and your old pet is not on vacation, it is dead.
HTTPS is far from perfect, but it’s much better than no HTTPS, and it is the only effective way to secure packages between upstreams and distributions. Now for an easy game: find an important free software package that is distributed upstream without using HTTPS. Don’t bother with small desktop software either, focus on big name stuff. You have a one minute time limit, because this game would be too easy otherwise. Ready, set, go.
Done? Think about how many different ways exist for an attacker to insert arbitrary code into the tarball you found. HTTPS makes these attacks far more difficult. Webmasters, please take a few minutes to secure your site with HTTPS and HSTS.