It’s free to try or demo the full version of course.
It’s free to own.
There is no buyer’s remorse, like when a new version of commercial software comes out and you now have an old version.
It’s free to upgrade. You can always have the latest version.
If you want someone else to have the data for editing, then they can download the application free.
If you work in an organisation then everyone can own a copy.
You can change pc’s and not have to worry about copy protection or licensing issues
If it is open source then you could pay someone or add features you want to the software yourself. Even if you’re the only one who needs those features.