"She sits down."
Translation:Sie setzt sich.
It's not optional in English in those examples. "I introduce" and "I introduce myself" have different meanings. Think of it similarly for the German reflexive verbs - they're incorrect or just different without the reflexive pronoun.
It can be tricky to remember which verbs are reflexive in German though, especially when they don't correspond to a reflexive verb in English. That's something I still struggle with too! Just need to keep practising...