Check that an email address is valid: format, MX record, domain existence.
Mailing lists decay by 20–30% a year: people change jobs, abandon mailboxes, and mistype addresses in forms. Sending to a dirty list produces a high bounce rate — the exact metric mailbox providers use to judge sender reputation. A few percent of bounces and your mail starts hitting spam even for valid recipients.
The validator checks the syntax of every address and catches duplicates and obvious typos. Run every new list through it before the first send, and re-clean your base every few months.