I'm going to go out on a limb and say he does not know what he is talking about when it comes to steam. Each game has a different location that it will install too. Some games will install the files under the steam folder some will install under the user's profile. There was a thread out on steam forum with a list of games and where all the config files are located to help people back up there game so when they did reinstall they still had there level 35 caster.
And what if a game writes to the registry I guess you can just ignore that all together.
I was talking about the game files themselves, aka what the OP is downloading.
Steam installs all your games within your Steam directory, e.g. C: \Program Files\Steam\
If you want save games, you have to copy from wherever the saves are located, but the OP isn't redownloading his saves, he's redownloading the games.
To "back up" your actual game files, you can just copy the entire Steam directory somewhere else.
Then, after installing your new OS, install Steam to wherever your Steam folder was put (e.g. Steam was on another drive, D : \. Install Steam on new OS to custom location, set the location to D : \Steam, Steam will pick up all your current game files and say your games are already downloaded.)
If there are registry entries required by the game, Steam will usually automatically sort them out as if you just finished downloading the game for a normal install.
I haven't seen any games that install to another folder, but then I don't have many Steam games. Maybe the settings/configs/save games might be installed elsewhere, e.g. My Documents, but the game files themselves...?
But again, per the OP, he is redownloading the games, and there was no discussion of save files.
As far as other games, these days many games can just be straight copy-pasted and then when you run the .exe from the directory it will just work.
Some require registry entries and will automatically add them, and some do want a reinstall to work, but many non-Steam games will just run straight off without a reinstall, in my experience.