To be perfectly honest, I've never used a single /command besides /init. That probably means I'm using 1% of the software's capabilities. In frankness, the whole menu of /-commands is intimidating and I don't know where to start.
The major benefit of agents is that it keeps context clean for the main job. So the agent might have a huge context working through some specific code, but the main process can do something to the effect of "Hey UI library agent, where do I need to put code to change the color of widget xyz", then the agent does all the thinking and can reply with "that's in file 123.js, line 200". The cleaner you keep the main context, the better it works.
Skills on the other hand are commands ON STEROIDS. They can be packaged with actual scripts and executables, the PEP723 Python style + uv is super useful.
I have one skill for example that uses Python+Treesitter to check the unit thest quality of a Go project. It does some AST magic to check the code for repetition, stupid things like sleeps and relative timestamps etc. A /command _can_ do it, but it's not as efficient, the scripts for the skill are specifically designed for LLM use and output the result in a hyper-compact form a human could never be arsed to read.
claude-code has a built in plugin that it can use to fetch its own docs! You don't have to ever touch anything yourself, it can add the features to itself, by itself.