MemoryInAGarden said:2) I didn't realize this at first, but the 64-bit version of IE doesn't support a lot of plugins. The icons are distinguished, but the browsers look identical in practice. This can be a nuisance if you use IE instead of another browser.
While I see it the other way around (many plugins don't support 64bit), I can offer a bit more info.
32bit mode is a per-process thing. Either a process is native 64bit, or it's 32bit and runs on WoW. Plugins are loaded into the IE process... So 64bit IE can load 64bit plugins and 32bit IE can load 32bit plugins.
Having a 64bit version of IE is a given, eventually all apps will go down that route. However, not being able to load one's favorite plugins would block customers from using 64bit Windows, so the 32bit version is included as well. Hopefull companies that produce plugins will start offering 64bit versions as well... I know it's a bit confusing, but the alternatives are not great.
That's not the only reason for having both version of IE, not by a long shot, but it's the one most applicable to this sceanrio.