One may not use all of the tools/applications mentioned in this article on daily basis, but these are some of the widely used applications in Linux administration and related Linux jobs.
Presence of numerous distributions of Linux, each having its own unique set of characteristics but, basically compatible with each other gives users' a variety. This allows users to select the versions which best meet their requirements.
In the proprietary closed source model, the entire development cycle evolves within a single company. Programmers write code, hide it behind binaries and charge the customers to use the software. No one ever gets to know how bad the software really is!