When to use an injector?

Can an experienced modder explain to me, when do I have to use an injection vs when tuning is enough?

For example, I created a custom CAS trait. The trait and its buffs work in the game without any issues and no injecting into CAS was necessary for the trait to appear in the game.

But then I added custom jokes. I added them under the game's pie menu Funny subcategory expecting that it would show up there but it doesn't. Is this when I have to use an injection to inject into the pie menu?

What is the difference in these two scenarios?

Unless the second scenario should have worked without injection but I didn't do it right - always a possibility. Even if so, I'd still like to know when an injection is necessary.