What does "precompile standard library" option mean in python installation?
When you import a python library and it doesn't have a corresponding .pyc file (or it's out of date compared to the source), python will "compile" new bytecode. It appears that this option simply does that for everything in the standard library so that it doesn't have to be done the first time you import them.
As to why it's not the default, I don't know; I'd guess it's because it has very little effect and some people don't want to spend the work on that for libraries they'll never use.