Steam runtime includes a lot of common static libraries that most games expect to be there. This means that older games will still work even though the wider OS might not have 32bit libs present.

You can start steam just fine without the packages. In fact, if you install without them, it'll ask you to install them every time, but you can skip that and it'll work, just 32bit games won't launch

Edit: Looks like I'm partially wrong, as pointed out by a commenter below, steam currently only launches the 32-bit version of the client, despite support for a 5l64-bit client