There are a lot of reasons for this- often dogs with go on people’s lawns or flower beds and this can kill the plants. Or, as my stoop in Brooklyn can attest, urine can actually make paint peel if many dogs are going on the same spot. And if you live in an area with a lot of dogs, you’ll probably notice the fingerpaint-like streaks of feces on the sidewalk since oftentimes the owner can’t get all of it up into the bag. When the dog uses the gutter, it minimizes the damages to plants and the cleanliness of the area.
Obviously this is one of those things where you just need to do your best most of the time, but it really will make a big difference.