The GUI is the most common UI by far. However, GUIs are expensive to create and iterate and frequently frustrating for the end user. These shortcomings could be addressed with better engineers and better product sense, but many companies can't afford or can't retain quality engineers, designers, et al. Does the GUI have too much marketshare? Are there other interfaces which are more useful?
Reasons why CLI is a great interface:
- All unix philosophy benefits, especially composibility, which implies network effects since CLIs' collective usefulness is an exponential or factorial function of the number of CLI tools available. You can pipe, grep, tail, curl any of your CLI outputs with any other CLI. (~50% of the benefit)