Asymmetrical (or contoured) ankle straps are shaped to follow the natural curve of your ankle and boot, usually with thicker padding on the inside edge where pressure tends to build up. Instead of pressing flat across your foot, the strap mirrors your anatomy for a more natural fit.
The biggest advantage is comfort on long days. If you've ever ended a session with a hot spot or numbness across the top of your foot, an asymmetrical strap can help by distributing pressure more evenly. You also don't need to crank down as hard to feel locked in, since the contoured shape holds your boot securely with less overtightening.
They're especially worth considering if you ride all day or have had issues with pressure points from standard flat straps. You'll find them mostly on mid-to-high-end bindings, and they're a solid choice if comfort ranks high on your priority list.