What is the greatest problem that FOSS has solved?

The entirety of web backends is built on open technology by default now. That's a pretty damn important achievement! Can you imagine if a company like Google or Microsoft was in charge or making our webservers or databases? It's really helped lower prices, increase accessibility and get people to contribute without being part of their organizations.

You can legally modify the program. You have access to the source code and documentation, so if you have experience or hire someone or ask a contributor, you can fix a bug or an annoying feature. An experienced programmer can modify or optimize or strip out the bloat of a program. The reason all of the clouds run on some variation of Linus is more the customization potential than the zero cost.

There are free, modifiable versions of all the expensive Adobe software, all of the expensive Office software, etc.

1. Software piracy

2. Keeping software projects as a going concern well beyond the lifetime of any single architecture or platform. We routinely run FLOSS code from the 1980s (~30 to 40 years ago!) on modern platforms, which would be altogether impossible if that code was proprietary!

3. An emerging problem: spyware software that "phones home" for user-hostile purposes.

It has commoditized some very expensive software at a low-low price of $0. This has reduced the cost of experimentation and unleashed a giant wave of creativity.