Precisely. Stuff on LW is posted unless some moderator decides to remove it; it's not brought to the main page unless some moderator approves, but it exists, can be read, and can be linked to.

Stuff on OB doesn't exist until a moderator decides it does.

If I've written something that I think the moderator will either dislike or be indifferent to, there's no point in sending it in to OB. Posting it on LW will get it seen and thought about, and if it's sufficiently popular, the moderators may even feel pressure to give it front-page access even if they dislike it personally.