A help center is usually organised by topic, searchable, and written for customers rather than for staff. Articles tend to be short and task-shaped: “How to change your delivery address”, “How to cancel your subscription”.
Why it matters
A help center does two jobs for an inbox. It prevents some email, because customers find the answer before they write. And it makes the email that does arrive faster to answer, because the reply can point to an article instead of explaining everything again.
It also becomes a source of truth. When a policy changes, updating one article is easier than remembering every template and past reply that mentions it. An outdated article, on the other hand, spreads the old answer to everyone who reads it.
An example
A customer asks how to download an invoice. The reply is two lines plus a link to the article with screenshots.
In ReplyPen: Your help center is one of the data sources ReplyPen reads to write a Draft, and the Check note names the article a reply relied on.