Well, it did improve someone's live, didn't it? I'm not claiming my library that has 3 stars on GitHub is gonna revolutionize all of humanity. But it's a little improvement. That contributes to having a more complete software ecosystem.
That code is now available to everyone that wants it. If they need it, it's there to use. Better than every company having to reimplements for the thousandth time the same closed software.
Which brings me to another point I didn't mention before. If a company uses an open source library, even if they are not required to publish their improvements to the library, they might do anyway because it is easier than maintaining their own fork and migrating every upstream change.