A number of reasons for the ladders. You can stand higher and spot fish better and get the cast out farther. When the fish move inshore they cruise right near shore or just off a shallow drop-off. It is more fun, and effective IMHO to spot the fish rather than just blind fishing all the time. The picture was on a nice spring day, but most of the season is in winter where it can be cold with strong Washoe winds. The more you are out of the water in the winter the better.
I have never used a ladder, but some have gotten elaborate. I was told it started with people standing on milk crates and then progressed. Now a lot of guys have nice fiberglass or plastic molded seats on the ladder. They stand to cast and then sit down while the line sinks and strip the flies back sitting down. Fishing with a nymph under an indicator has become very popular and you can do that sitting all the time. You set up on a point with a wind driven current, roll cast the nymph and indicator upwind, let it drift until your line straightens and repeate endlessly. That drives some of the purists crazy who think just sitting on your butt is lazy.