Having the source lets you fix something for yourself, there are an increasing number of barriers being put up to prevent you submitting a fix upstream.

Going through this right now with part of libpng, their mailing list doesn't seem to like my email.

Using a source-based distro (previously Gentoo, now NixOS) lets me solve the problem for myself, even if my PR never gets accepted. Right now the count is at 4 patches in software I use that I submitted upstream that were (for one reason or another) never accepted.

In at least one case, I later found out that I was not the only person to submit a fix for the problem I was running into, but their discussion on the ML also went without comment 3 years earlier.