Not sure what that file does but it is like the quick fix for alot of things. Delete and re-start STEAM and it comes back and it ifxes most things. For example I had a problem with games going AWOL from my list and that was how I fixed it.
its filled with alot of random stuff. I believe if you delete it, it makes steam think it needs to be updated which when you start it up steam will update
actually
The Clientregistry.blob file saves some account-specific settings (corruption of this file or outdated information may interfere with installing and updating Steam games). This file can be found (by default) under your Steam installation folder.