CLIs have a discoverability problem. Knowing how to use any given one is a skill that takes years to hone. Learning a single command can take up to an hour. Most webapps are more constrained, but a common operation can usually be accomplished by someone who hasn't seen it before in less than 3 minutes.
The 'apropos' command is very handy on machine or OS you aren't familiar with.
If you want to find out what tools are available on the CLI that operate on jpegs, you'd try 'apropos jpeg' and get a list of things that mention jpeg in their man page.
