Easier to grip. Cooler water.
The opening of the bowl is 4.6" to make it easy for your pup to drink. The same wide lid can be hard for smaller hands to twist shut. The sleeve gives you extra grip allowing you to get the lid tighter.
The bowl is made of Tritan and the slick surface makes it easy to clean. On a hard floor, a tailgate, or a smooth rock, the bowl can sometimes slide. The grippy sleeve keeps it put.
It insulates, too. On the hottest days the water stays cooler longer.
Pull it off to wash it. It can go through the dishwasher with the bowl. 100% BPA Free.