A wide velcro power strap (40mm or wider) gives you more surface area for contact, which translates to a stronger, more secure closure around the top of your cuff. Compared to standard 30-35mm straps, the extra width means less stretch and better power transfer from your leg to the ski—so when you drive your shin forward, more of that energy goes directly into your turn.
This upgrade makes the most sense if you're an aggressive or advanced skier who wants responsive, precise performance from your boots. If you're mostly cruising groomers at moderate speeds, a standard strap will do the job just fine.
One thing to keep in mind: wider straps can be slightly harder to adjust on the fly, and they're typically found on performance-oriented boots anyway. But if you're between models and one has the wider strap, it's a nice feature that adds a bit of extra control without much downside.