As this backdoor has shown, extra unnecessary files in the source files can make it easier to hide malicious code. If you take Gentoo as an example, when a software package is built, Gentoo creates a sandboxed environment first, disallowing the build process from impacting the rest of the operating system.[1] Removing superfluous files from the source tarballs minimises the ability for an attacker to get malicious code inside the sandboxed build environment.

Sandboxes for building software are commonly used throughout Linux distributions, but I am unsure how strict those sandboxes are in general e.g. whether they use seccomp and really tighten what a build script can get up to. At least on Gentoo, there is a subset of packages (such as GNU coreutils) that are always just assumed to be needed to build software and they're always present in the sandbox. Build dependencies aren't as granular as "this build needs to use awk but not sed".

[1] https://wiki.gentoo.org/wiki/Sandbox_(Portage)