So you can inspect, adjust or learn from it
How can I view the source code of an exe file?
When developers release their software as open-source, it shows that they are both trustworthy and generous. Open-source software was one of the most important things that helped me personally get started as a self-taught programmer.
To make a project open source, the developer must make the source code publicly available so that anyone interested can inspect and learn from it. Importantly, it also shows that the developer has no malicious intent, since open source code allows users to compile their own binaries instead of using the ones provided.
If you ever come across two pieces of software and can't decide which one to use, check to see if one is open source and the other isn't. If so, your choice should be clear.