The commercial players have mostly abandoned this approach, though, in favor of technologies that render on the client. They're more performant (tiles are smaller), and necessary if you want, say, to be able to rotate your map and still have all the labels be upright and not collide, or show/hide individual geographic features dynamically in response to user input, or apply perspective/tilt as you would for a heads-up display in a car, etc.
Mapbox GL was Mapbox building that (as already exists for Google Maps, Apple Maps, etc.). At a very high level, it and Leaflet serve a similar purpose, but are technically very different from one another.
(full disclosure: former Mapbox employee, still a shareholder)
