It makes it simpler for some users. If you don't know whether you're on 32-bit or 64-bit Windows, or you're not aware of the fact there's a difference, the installer picks the right one for you. It also picks a good place to install the app if you're not an admin, so if you don't know where to put the files it chooses for you, and creates shortcuts for you. That might sound a bit patronising, but not all users are tech-savvy enough to know good answers to those questions. Remember Construct 2 is aimed at "beginner" users. Given it's alpha, we're only likely to attract tech-savvy early adopters at the moment, though.
Either way with zip or installer it's only a few clicks to have it ready. Is it that big a problem? It's still a portable application, so if you like, you can install, zip up the folder you installed it to, and take it where you like. (But I still think it would be better to take the setup with you, since it works out 32/64-bitness and whether admin privileges are available.)