So you can access things outside, helps in docking, getting up to the bow, throwing bottles at seagulls, whistling at bikini babes, all kinds of things
Probably if you're clever you can just swing around in the chair and take a leak over the rail. All kinds of things. I wouldn't suggest attempting that while underway or in a crowded marina but to each their own.
Yes, if there was water that was around due to heavy conditions, rain and other storms or some schmuck wizzing out their door as you pass by there would be some moisture, but generally it's closed up and there are tracks, guides and enclosures to ensure the operator doesn't get much (if any) water through there.
I figure people who have their boats on a mooring and run single handed wthout a side door, have to run out of the cabin to the bow or stern to grab the mooring. Either way, you can't see it and by the time you get there, the boat will have drifted away. With a side door, you can see it and pull right next to it. Tying up to dock, again a lot easier to step out with a line than scramble aft through the cabin door and get off before the boat has drifted away.
My stb side hinges up instead of sliding.
1st thing I did when I bought the boat.
At 1st I planned to do a slider but figured if I'm going to cut and open up the side, I'm really going to open it up.
It has to be really nasty or cold out for me to close it while underway.
I fish out the stb side right at my helm most of the time.