Although the question about Ubuntu's key advantages is very subjective, I'll try to name a few points. The list can of course not be complete and not everybody might agree on every point, but it's a start:
- free, open.source operating system and standard software
- huge central software repository with checked and verified software
- PPAs (private package archives) that allows every developer to provide packages for Ubuntu in an easy and user-friendly way
- freedom to also install software from external sources (although not recommended for security reasons)