It's less that is _can_ and more that it _has to_ in order to be optimal. The launch window pretty much guarantees that if the rocket launches, and stays on a

It's less that is _can_ and more that it _has to_ in order to be optimal. The launch window pretty much guarantees that if the rocket launches, and stays on a course that is designed to get it outside of earth's atmosphere using as little fuel as possible, it will meet up with the orbiting space station "by chance". Launching outside of this window means that it would have to spend fuel to realign itself and go find the space station on it's own.