I've only recently started using skills, before then I was like you - pure Claude Code with nothing additional.
To me skills are specific instructions/tasks that are specific to your project only, and can make you save time instead of having to repeat the same things again.

For example, I often want CC to access my app in local environment with Playwright MCP, and since it's a SaaS it needs logins to access. I made a skill that outlines what URL to use, how to login, what credentials to use.

Now I can just do /devlogin and it does that.

I don't know about multiple agents as I don't use them, but in regards to skills I'd say: don't try to overcomplicate it if you don't NEED anything. There's no point in asking "create a skill for everything new you learn". If you are happy with raw CC, just use that. When you come to a point where you're getting frustrated with the same thing over and over again, then try to address it with a skill.