If you’ve ever selected the Explorer’s Pack, or the Burgler’s Pack, or if you simply stumbled across them while looking at equipment, Pitons are a bit confusing if you’ve never gone climbing. Because who goes climbing during Covid (probably quite a few people actually)?
In real life, Pitons are small metal spikes that have a single hole called the Eye for threading your safety line. In DnD, they’re pretty much the same. They cost 5 copper (at standard prices), weigh 1/4th a pound, and should easily fit in any of your packs.
What you can do with Pitons, is if a wall doesn’t have enough holds for you to climb, you can use the “Activate Item” action to create your own by spending these pitons, though you’ll probably need a hammer to do that, and two free hands, unless your strength is ridiculous. If you have a Climber’s Kit, you can actually use the “Activate Item” action to anchor yourself, which keeps you from moving more than 25 ft. away from your anchor point without undoing said anchor.
Now some ideas for what you can do with Pitons, include jamming them into an enemy’s back so you can hang on while they’re thrashing you around, setting up an escape route for a cave, climbing down a deep drop without using your rope, bracing yourself for an attempt to push you (in my games that would be advantage on the required Athletics check), or even having something to hold onto in the event of a Reverse Gravity spell. It may not seem useful, but even mundane equipment in 5e is incredibly useful in the hands of a resourceful player. If Pitons are this useful, then imagine what you could do with a bag of ball bearings, caltrops, sand, a crowbar, grappling hook, rope, mirror, pick, shovel, whistle, soap, oil, acid, or other various supplies! Just watch out for that bottled ooze in the mimic.