Transactional email is sent in direct response to something a user did or something that happened to their account: a signup confirmation, a password reset, an invoice, a shipping update. It's expected by the recipient because they took an action that triggered it.
Because transactional mail is time-sensitive and expected, it's typically held to a stricter reliability and latency bar than marketing mail, and is the strongest reason to isolate it onto its own sending lane so marketing-related reputation events can't delay it.
The distinction from bulk/marketing email matters for both deliverability (different lanes, different reputation) and compliance (different legal basis for sending under most consent frameworks).