>>105789906All email communication happens via SMTP and IMAP or POP3 protocols. What you see in a browser is just a web-based SMTP/IMAP client, just like Thunderbird.
So what you just asked was "why do I need a native client when I have a browser-based client?" and the answer is simple: efficiency, speed and feature set. Thunderbird allows you to do much more than typical web-based clients. Also you get to keep offline copies of your mail with POP3 and and you can answer your mail queue while offline and send everything when you're back online, e.g. while taking a flight somewhere. Obviously there are other nice features, too, but if you're happy with your mail provider's cucked web interface, so be it.