Most visitors are not ready to buy. They compare, hesitate and explore, leaving no trace today and forgetting you tomorrow unless you provide a good reason to leave an email address.
A lead magnet does precisely that: a useful guide, checklist or calculator exchanged for an address and follow-up permission. A small detail matters: email-only forms convert at 4,41%, falling to 2,90% with a second field and 1,93% with a third. Your magnet is only as strong as the proposed exchange.