I have been using open source software since I started my first job around 10 years ago. By then the world was very different, Microsoft still dominated most of the software world (Windows, Visual Studio, .NET Framework, MS Office, SQL Server), and open source was still a small part of the whole ecosystem. Yet open source spread up like a virus over the last 10 years, and now even Microsoft is forced to embrace open source (well, it's also CEO related I guess).
To me, there are two major advantages using open sourced software:
free of charge
able to inspect, modify and recompile the sources.