I'm pretty sure it was about 20 years ago that I saw my first set of locking tuners. They were on someone else's strat, and they had me a bit baffled, to tell the truth. Up to that point, I had guitars that were older, (the newest was my 95 Les Paul) and all the tuners I had were just straight-up standard style.
There are a bunch of different, effective ways to lock a string into place on a standard tuner to avoid slippage...but locking tuners win out for some folks on a couple of different levels.
For one, changing strings can go so much faster with a locking tuner. Especially if you broke a string during a gig, and only have a 20 minute set break, just locking a string into place, tuning it up real fast and stretching it out a few times gets you back into the action in a snap.
Another great thing about locking tuners is they take the guesswork out of re-stringing a guitar. The functionality is easy and simple, and you have a better chance of avoiding string slippage.
One super popular type is the rear thumbwheel locking tuner, some folks call them a "coin" tuner. You just place the string through the post hole with a little slack, roll the thumbwheel down to lock the string in place, and tune as normal. There's a ton of major manufacturers that make these, and you can get them at multiple different price points, but the engineering of them is essentially the same.
Another type...and these are less well known, are self-locking tuners. There's a version of these used on some PRS guitars (that I have been told. I've never happened to touch a PRS guitar). These have a reverse-threaded post mechanism. You place the string through the post hole with a little slack, and then simply tune it to pitch. The post, (reverse wound) will wind down and automatically lock the string as you tune. Very cool and simple. They also weigh less and have less mechanism than thumbwheel tuners.
Here's a video of the two types in action so you can see the differences.
We sell our own versions of these in our shop, and they receive rave reviews. I test them out on my guitars as well and believe me, I put them through a lot! They actually tend to sell out pretty quickly, so if what you are looking for is sold out, just ask us for an ETA on re-stock. Check out our tuners for sale here!
