What it's for
Port 993 is IMAP wrapped in implicit TLS — the encrypted variant of port 143 that virtually every modern email client defaults to. It provides the exact same folder-synced mailbox access as plain IMAP, but guarantees the connection is encrypted from the very first byte, rather than relying on an optional STARTTLS upgrade partway through.
Protocol basics
Functionally, port 993 speaks the same IMAP protocol as port 143 — the difference is purely at the transport layer, with TLS negotiated immediately as part of the connection setup. Because the handshake happens before any IMAP commands are exchanged, there's no window where credentials or mailbox data could be transmitted in plaintext, which is why most mail providers and clients treat 993 as the default, expected port for IMAP access today rather than treating STARTTLS-on-143 as equally standard.