Comment The thing that's nice about this (Score 1) 135
is that it allows the package manager to co-install packages of two different architectures in certain cases. This means that you can install a 32-bit Firefox (if you have some proprietary plugin) and have the rest of the system be 64-bit. Or you can install most of the packages from the armel port (ARM EABI soft-float) and install floating-point intensive ones from the armhf port (ARM EABI hard-float).
Previously, in order to install any meaningful amount of i386 software on an amd64 system, you had to install a package called ia32-libs, and if it didn't have the library you needed, you were SOL. Now you can install i386 libraries in parallel.
This is how it works in theory. Not all packages will be updated to be multiarch aware immediately, so YMMV.