I may not read the source code of every tool I use, but even if the average user doesn’t read the source code, having it available for inspection by others in the community increases security, trust, and overall software quality. All a user really has to do is look at the license of the software they use, typically a GPL or similar license, and consider how reputable it is. Not only that, but if you’re on Linux already, you can just get most of the software from your distro’s repositories.