Several reasons. First and foremost they make oil changes a breeze to do without getting a drop of hot oil on your hands....... Or anywhere else for that matter. More importantly, on a lot of the newer cars, they have plastic, or polymer oil pans, with a threaded metal insert molded into them that contains the drain plug.
On my 5.7 HEMI the plug itself can become very difficult to remove for some reason. Yes, even using recommended torque. I've even used anti seize on the drain plug to no avail. Removing it can take a lot of torque. This is a widespread issue, that appears on a lot of the Jeep forums with these crappy plastic oil pans.
It's only a matter of time before it gets stuck to the point of turning the insert in the pan itself, causing a leak. The fix is a new $1,500.00 oil pan installation with labor. Only to have the same issue repeat itself in time, because you're going to have the exact same condition.
With a drain valve system once you install it, you never touch a wrench again to change oil. It makes the whole process much easier and safer with these wonderful new, "improved" plastic oil pans..... (The picture shows the molded in insert with the drain valve system installed).