1) To be able to assign tags based on rules
2) To be able to view those tags in IMAP clients (Gmail exposes labels as IMAP folders)
3) To be able to Archive mail so they are no longer in the Inbox, but so the tag remains (in Gmail, 'Inbox' is just a tag that is removed when you Archive mail)
Zoho fails because:
1) Tags aren't anywhere near a first class citizen. You need to hover over an icon to see what tags are assigned to an email.
2) There's no way to search Archived mail (yes really) and it's not available over IMAP