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You missed out on the Browser war between Internet Explorer and Netscape... Lots of features on the net in the 90's were compatible with either Netscape only or IE only. So a lot of code (js) used the user-agent string to disable features for incompatible browsers.
As IE gained dominance and other browsers emerged they needed to outwit these limitations and so they started to "pretend" to be something else. Hence IE5+ have Mozilla 2.0-4.0 in the user agent string. It's a false ID basically to fool disabling code.
Reminds of that horror Hotmail did a while back in creating a full page warning about using Opera or Mozilla... . Some browsers actually support the ability for the user to change their user-agent string as a preference.
Well, you can't trust the user agent at all anymore. Some of the newer browsers allow you to change the user agent to anything. And you could go through some proxies that change the user agent.
With Opera you can spoof the user agent so you can look like any web browser out there.