Why Open Source?
The 4 Freedoms of open source software are a radical departure from conventional, closed source models. These are the freedoms to inspect, use, modify, and sell.
1: The freedom to inspect or view. In software, this came from the freedom to study how the program works, and change it to make it do what you wish . Access to the source code is a precondition for this – such as FreeCAD files, text documents, spreadsheets, calculations, instructions, etc.
2: The freedom to use. Run or otherwise execute the software, product, or process.
3: The freedom to modify. This is a big point: making improvements or adaptation is a key to distributing value.
4: Economic freedom. The freedom to sell. Freedom distribute copies of your modified versions to others. By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
Fun Fact: Before large companies started creating and selling their own software, most programs were actually open source!