A form submission on a marketing website, a lead captured through a Facebook or Google ad campaign, or a badge scan at a trade show all generate the same thing: a prospect's contact info sitting in a system that isn't Odoo. Someone has to notice it, copy it over, and create the lead manually, which is both slow and the kind of step that quietly drops leads when volume picks up.
A related gap shows up on email marketing. Odoo CRM tracks the sales side of a relationship well, but if marketing nurtures prospects through a separate email tool before handing them to sales, keeping both systems aware of where a lead actually stands usually depends on someone updating two places instead of one.