Hum is able to both send and receive emails. People who receive stuff you send over email will be able to reply using the usual reply button, but it's not a full-fledged email client. The email integration is there to make it easy for folks who'd like to use it with their existing network of email contacts.
In addition, every Hum user gets an email address at letshum.com that they can use for incoming email.
Correct me if I'm wrong, but can't I open Gmail, send someone a message (@letshum.com or @facebook.com), and they receive it as a Hum or Facebook chat message? Then, they can reply to the message in their chat client as if it were a text, and it goes to my Gmail? That seems quite similar, and in both cases I can communicate using email, while my friend uses chat. This seems to be the underlying reason why both services give every user an email address, so I fail to see how this sets Hum apart from other chat apps.
Facebook has announced their intention to retire @facebook.com addresses, forwarding mail sent to them to the user's primary email address and making all chats involving @facebook.com addresses read only.
If I may, linkedin does allow users to correspond with each other using both mail clients or directly through their platforms. The part that I am not too sure about is how does Hum solve user problems that whatsapp/wechat can't. The gap I find as a user is that unless there are ways for Hum to integrate with the fragmented contact databases we have across the various social networks (i.e. my gmail, 1k+ linkedin contacts, facebook friends, twitter correspondence etc) and make my life easier, I may not venture and try yet another chat platform.
In addition, every Hum user gets an email address at letshum.com that they can use for incoming email.