The first comment is a comment the brand adds to its own post as soon as it goes live. On Instagram it is often used to keep hashtags out of the caption, so the caption reads cleanly; on LinkedIn and Facebook it can hold a link, for teams who prefer the post itself without one.
Posting it by hand means being there at publishing time. A scheduler can post it right after the post, so it is the first comment people see. Whether it changes how far a post travels is debated; what it reliably does is keep the caption tidy.
