I'll give you a practical reason for having a browser, apart from the obvious one that is.
I have a friend I chat to online. We used to use MSN, but since that service stopped, and she can't get on with Skype, we chat using a supposed social networking site.
Now, rather than having my usual browser running with one tab reserved for chatting to her (if, that is, she happens to be online), I run a second browser logged with the social networking site open. That way I can use my default browser for doing all the things I might want to do, can open and close it as often as I like without constantly having to go back and log in to the social networking site.
Now, sure I could close tabs as I go along (and often do) leaving the social site tab there all the time, but I prefer to have it open in another browser and to be able to shut down my main one as and when I feel like it. Other people who use this computer also have their preferences - mostly they prefer Chrome whereas I prefer Firefox, and nobody here likes IE very much.