The reflexive version just describes that the person that does the action is also the object of the action. So setzen comes always with an object or a reflexive pronoun (sich/mich etc.) as a "replacement object".

Ich setze mich auf den Stuhl. Ich setze das Baby auf den Stuhl.

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...