Why play Sword & Backpack? What do you gain vs. something like 5e or Shadowdark RPG? What do players experience that is different?

As with any ttrpg, there are a lot of overlaps in terms of how you play - making decisions as characters, rolling dice for outcomes, with a reactive world supplied by a specified player. 

But this is what sets Sword & Backpack apart:

1 - character narrative controls skill and ability, not numbers and assigned stats. Sword & Backpack is not just rules lite, it is math lite, and story heavy. This makes it great for players who find the math, character sheets  and stat-character builds overwhelming.

2 - it is deadly and your actions have more serious consequences. Sword & Backpack is fairly simple in that if you intend to do something and roll well, you do it. That includes killing in combat, making risky moves, and failure is perilous. Doing something risky or getting in a fight could get you killed in a single turn. So players have to be careful, clever, and lucky. 

3 - Sword & Backpack is more about people on adventures, less about people becoming powerful. A leveling system like 5e leads to a gameplay focused on exercising, gaining, and maintaining power. S&B is about gaining experiences and using your experiences to overcome obstacles and solve complex dungeon, wilderness, or societal problems. It is more about becoming wise and crafty than it is about becoming strong and authoritarian. 

If any of that sounds interesting or refreshing, check out Kaznath's Hack - as well as the original editions of Sword & Backpack by Rothbard & Gazpus. They are all quite free.