Here's a typical scenario – you install software from source, spend an hour figuring out the configure options, and finally run:
./configure --prefix=/a/b/c --with-X --with-Y \ --with-Z-dir=/usr/local/Z --enable-A --disable-B
Years pass by and for some reason you need to update the software to a newer version. To do that, you need to run configure again on the new source code, however, you have forgotten the configure flags because it's been a long time.
If you're lucky, you still have the old build somewhere in your home directory. Here's what you do. You go into the old build directory and run this:
./config.status --config
This will output the exact configure arguments that you used to configure the software. You just saved yourself an hour of work trying to remember all the arguments.