The very minute you pick up your first XD pistol, you notice the funny little button/lever in the back of the grip. Is it to help take off the backstraps to replace them? Is it a loaded chamber indicator? Does it have something to do with take down disassembly? No, no, no, it's an external grip safety and it's been around for a hundred years give or take.
Why a grip safety?
(this is why the grip safety was demanded by the US Army in the 1900s)
Grip safeties on semi-automatic handguns have been around for a long long time. They exist to blocks the trigger from firing and require grabbing the gun to disengage. Going back to John Moses Browning's Colt 1911 design, on the near vertical cant grip of that famous gun, is a grip safety. The Colt was a single action .45 that was designed for the Army to issue to cavalrymen (back in 1911 the Army still had 14 full regiments of good horse mounted cavalry, and even a few Indian Scouts to boot). If you are galloping into battle on the back of your horse and reach into your flap holster to pull out your cocked-and-locked .45, it's nice to have some safety on it to make sure it goes off with your finger just barely on the trigger until you get a good grip on it.
By placing the grip safety high on the rear backstrap, its almost impossible not to deactivate it if you have are holding the pistol properly. Fewer accidental discharges, fewer perfectly good cavalry horses not shot in the grape by their riders, everybody is happy.
"That's the stupidest feature ever!" come the cries of diehard 'tactical' shooters. There are loads of experts who have endless smack about grip safeties that include: This little lever can keep you from being able to engage threats in a timely manner. Having to adjust your grip to depress a safety can throw your shot off. If you have to shoot in an unorthodox manner, such as from behind cover under your car while your hand is propped up on a fallen Tango, you may not be able to pull off the grip safety. Its best avoided by all serious shooters, after all, if you are safe and follow the four basic rules of firearms handling, who needs an active safety!
Why not a grip safety?
The XD was originally the HS-2000, designed as a combat pistol in Croatia for their military. The inventors looked at several successful combat pistols over the past century and borrowed as many things that worked well as possible to create a fully evolved platform. From the Colt 1911, they borrowed the grip angle, long sight radius, and...the grip safety. If it was good enough for a handgun that has seen more than a century of use by the US military and legions of competition shooters, why not?
Grip safeties add a layer of positive control to the gun itself. Like the horsemen of 1911, the XD owner of a hundred years later is required to have a good and effective grip on their handgun before the safety disengages and allows them to pull the trigger. It's a good practice. Let's role-play this out. Say you live in a home with an inquisitive child who somehow finds your loaded yet unsecured pistol. If said pistol is a SIG, a Glock, a Ruger, etc., it's as simple as pick it up, pull the trigger, and you have a problem. With an XD, they have to fully grip the gun to depress the safety lever on the backstrap, and then pull the trigger, which may be harder for small hands.
Of course, every firearm should be secured around children, and every child should have some sort of basic gun safety training, but it's nice to have that little extra lever out there.