TL:DR?
It's a pointless styling accessory. Don't worry about it.
She will have a far more secure ride either reaching somewhere around the small of her back and gripping the backrest uprights, reaching around and hugging you (no funny business here - This is a family forum and your focus should be on the road!), or simply keeping her hands on her knees and relaxing.
The pillion strap does absolutely FUCK all in terms of... well, anything that I can imagine.
On a pillion pad like that, it's bloody awkward to get even one naked hand under it, let alone two gloved hands. Heck, I have a tight squeeeze just getting the pad back under it when refitting, let alone someone actually using it while riding.
Dynamically, it's pointless.
Try it - Sit atop a high stool, grab the front edge and lean back - Notice two things:
1/. Your chosen anchor point is pretty much the same as your pivot point, which does nothing to secure your centre of gravity - Your upper body still tilts rearward to the point where you will fall off the back, feet flailing in the air.
2/. If you do manage to get in a suitable position, every time your body tilts back, your wrists crush your nuts to hell!!