I've been reading the Reputation help doc and my question is:
If you earn rep by helping others answer questions, why do you have to have 50 rep to do it? I don't seem to get this.
You need 50 reputation to comment anywhere. This is different from asking or answering questions. Everyone can provide an answer by writing it in the big answer text box at the end of each "thread". But if you want to comment on someone's answer, for example because they have forgotten something in their answer that you noticed and you would like to help them improve their answer, that's what you would use a "comment" for.
New users can write comments under their own posts, such as their questions and answers, for example to reply to comments from other people (be sure to incorporate the feedback you get into your posts by editing them). They can even write comments under every answer to one of their questions, for example to clarify what they were asking and what an answerer was missing. But they can't write comments under the posts from other people.
You get reputation for upvotes on your questions (+5 each) and answers (+10 each). Upvotes on comments don't give you anything.
Questions and answers have an extensive revision history that allows anyone to see all changes. They are also never truly deleted. People with more than 10,000 reputation or moderators can still see them. But when a comment is deleted it is simply gone (technically mods can un-delete them, but for practical purposes they are gone). Comments are ephemeral post-it notes that are supposed to point out problems in questions/answers and they are unlocked once you reach 50 reputation by asking/answering questions or suggesting edits (+2 for each accepted edit).
You can find an overview over the different privileges here.