Crabs walk 'sideways' because of how their legs bend - they don't bend forward (like the knees on you and me)!
Sideways-bending legs enable them to fit into really small holes and crevices. If they couldn't hide so well, they'd get eaten pretty fast by something else. It hardly slows them down, either!
Crabs and Vision
Crabs can quickly see where they are going, they can see 360 degrees around themselves - they have little eyes that act like periscopes. In fact, crabs have a kind of reverse tunnel vision. Their vision is blurriest in the centre of their field of view, and sharpest on the edges. Lines and borders stand out extremely well to crabs, bettter than other features.
Crabs vs. Lobsters
Lobsters, by comparison, walk forwards --- they don't spend so much of their time hiding or looking for somewhere to hide. And their vision is nowhere as good as the average crab's.
Crabs vs. Lobsters
Lobsters, by comparison, walk forwards --- they don't spend so much of their time hiding or looking for somewhere to hide. And their vision is nowhere as good as the average crab's.