Want to know why? The default lid-closing action for most laptops is to put the system in "sleep" mode, and Windows is notoriously bad at waking up properly.
That's why I advise most laptop users to use "hibernate" mode instead, as it's much more reliable when it comes to waking up.
You see, sleep (a.k.a. standby) puts your system into a low-power, off-like state, allowing you to pick up where you left off (in theory, anyway) after just a few seconds. However, a PC in standby mode continues to consume battery power, so it's not uncommon to return to a "sleeping" PC to find that it's just plain dead. Or, in my aunt's case, unresponsive.
Hibernate, on the other hand, saves your machine's current state to a temporary hard-drive file, then shuts down completely (much like "off"). When you start it up again, it loads that file and returns you to where you left off--no booting required.
Both ends of the hibernate process take a little longer than sleep mode (usually 10-20 seconds, in my experience), but you avoid any of the issues that can arise when Windows suddenly loses power.