SSL establishes a secure connection channel between you (the client) and the web server. If your only worry is about someone sniffing your traffic and reading your communications, then this is good enough.
However, this implies that you must place your trust in the web server. Your chats will be stored here (e.g. HTTPS protected conversations in Google Hangouts). However, this also means that your messages can be read by the server.
What end-to-end encryption (e.g. WhatsApp) gives you is that only the involved parties can access the details of a communication, not even the provider (more details). Session setup now requires keys that only the two parties know, without any trusted source (e.g. in SSL, the browser trusts the root).
The situation gets even more complex for group chats (see discussion here). Now you need all parties involved to agree to a key, and you may want to revoke and create new keys every time group membership changes.