The form is not asking for a way to contact you
When a website demands an email address before showing you a price list or letting you download a file, contacting you is rarely the point. Your address is an identifier - globally unique, rarely changed, and typed identically into every service you use. That combination is what makes it valuable: it is the join key that lets separate databases agree they are talking about the same person.
Hand the same address to a shop, a newsletter, and a forum, and any two of them - or anyone who buys, shares, or steals their data - can merge what they know about you. That is the real transaction behind "enter your email to continue."
Where your address travels after you type it
The address rarely stays where you put it. It flows into marketing platforms and CRM systems, gets hashed and uploaded to ad networks for "custom audience" matching, gets enriched by data brokers who attach names and histories to it, and sooner or later sits in a breach dump when one of those systems is compromised. Each stop adds a copy you cannot recall. Deleting your account at the original site does not chase down any of them.
And unlike a password, you cannot rotate your address after a leak without breaking every legitimate login attached to it.
The spam is the receipt
The mundane cost shows up as spam: once an address is confirmed live - say, by an email tracker registering an open - it becomes more valuable and circulates more. The compounding cost is quieter: profiles about you get more complete every time the same identifier appears in a new dataset. You never see this happen; you just get eerily well-targeted ads and the occasional extortion email quoting a password from a 2019 breach.
Breaking the join key
The fix is to stop using one identifier everywhere. For throwaway interactions, a disposable email address is the cleanest break available: the address works for as long as you need it, then you burn it. It cannot sit in a breach dump usefully, cannot be enriched, and cannot join anything - there is nothing on the other end.
For services you keep, the same principle argues for compartmentalization: a distinct address per relationship, so no two databases share a key. That is what Mail Vanish Private Emails is for - permanent addresses you own, one per service if you like, with forwarding to your real inbox when you want it.
A fair rule of thumb
Before typing your real address into a form, ask what the relationship is. If the honest answer is "there is no relationship - I want the thing behind this form," give them an address with a lifespan to match.