The visitors who quietly disappear
Here's an uncomfortable truth about websites: the large majority of people who visit yours are not going to buy on that visit. They're researching, comparing, or just not ready yet. On most sites, every one of those people leaves and is never seen again — an anonymous visitor who came, looked, and vanished.
A lead capture system exists to rescue those people. Instead of letting an interested-but-not-ready visitor disappear, it offers them a reason to leave their contact information — a name and an email, maybe a phone number. Now they're not a ghost; they're a lead. And a lead can be followed up with, reminded, and eventually won.
This is the single biggest leverage point in most funnels. You already paid (in time or ad spend) to earn that visit. Capturing even a fraction of the people who weren't ready to buy multiplies the return on every visitor you attract.