Someone finds your site, reads a page, and leaves. Nothing about that visit gives you a way to reach them again. It adds up quietly, because the visitor who was interested but not ready is exactly the one worth a second conversation, and there is no way to start one.
A signup form fixes that, but only if it is actually on the page. Send29 gives you two ways to put one there: paste a single HTML snippet into any site, or share a hosted link if you don't have a site yet. New subscribers flow straight into your lists, with no exports to run.
Interest becomes a contact you can reach on purpose, at the moment it happens rather than after it's gone.