So this was a problem with eggs too.
Let's say ZODB 3.0.1 was just released. You are happily using 3.0.0, the next version is a minor upgrade, but there are no precompiled packages for 3.0.1, so your build breaks on Friday morning when you are trying to deploy. Compiling it yourself is not an option because you are on Windows XP, and the compiler is so hard to install that you would need to hire a consultant to get it done; only to bridge the 1-week gap between a new ZODB release and the precompiled eggs for your platform.
If you find yourself in that situation then you would appreciate an easy way to use the newest available binaries rather than the newest available source, or perhaps to install directly from your cached binaries without going through the package index. Manually pinning the older version just to get the binary is going to ruin your day.
With wheel we can at least approximate this by doing a two-phase install, first dumping all your project's dependencies as wheels to a wheel directory, and then only pointing the installer at the wheel directory for the deployment.
Daniel