There are two main reasons:
1.) To start a conversation with a Steem Account.
Transfers are getting used by many Steem users but mostly without encryption. It's easier to get in touch with a Steem user through transfers (messages) rather than to try and find the steem.chat or discord name. And the messenger automatically decrypts and encrypts messages for you - so there is no reason to not use encrypted messages.
The conversation can of course be switched to another plattform like discord after the initial few messages.
2.) To easily send transfers to promote your posts e.g. Smartsteem.com
It's really simple to send transfers for promotion of your posts with the messenger. Smartsteem.com also supports encrypted memos by the way ;)
Thanks!