There's now a standalone dog walking service under rates, but there's also the previous one under "sitter's home"
This is a good point that you may want to email to support@rover.com. My interpretation is that the walking rate is listed in multiple places, for the convenience of pet-parents to easily see that option. It makes sense that it's a standalone rate listed at the bottom under Dog Walking (done at the client's home) and also listed under the heading House Visits. (again at the client's home if they're considering walks vs. stays) However, I'd think it doesn't really apply to the Dog Boarding category. My thoughts are that walks at the sitter's home should be included (not charged separately) under either the day care or nightly rate. It seems extremely rare that a pet parent would drop off & pick up their pet from a sitter's home for just a walk.
I'm not sure if they're planning on eliminating the dog walking charge under the boarding category or not, but before it was a separate option under travel sitting it was convenient for boarders who weren't able to (or interested in) providing travel sitting services to offer clients walks as an option. It can still be used this way, I suppose, but I mostly see its use as a way to up-charge for particularly high energy guests who require more exercise than a boarder would typically provide in a day so you get compensated for the extra work required for high energy dogs.