When Shimano introduced Hyperglide sprockets , one of the new features was "ramps" cut into the sides of the sprockets to facilitate shifting.
The ramps help to catch the side plates of the chain to lift it up when shifting to a larger sprocket. This worked so well on rear sprockets that it was later applied to chainwheels as well. Since chainwheels do their shifting with the tight segment of the chain, they are subject to more stress in shifting. In addition, chainrings are generally made of aluminum, which is softer than the hardened steel used on rear sprockets. Ramps on chainrings would get worn down easily and loose their effectiveness.
To get around this problem, steel pins are commonly riveted into the sides of chainrings at the point of greatest stress.
Ramped/pinned chainrings generally give improved shifting when moving from smaller to larger chainrings. The ramps and pins are always located on the side that faces the next smaller chainring. Ramps and pins are useless on the smallest chainring of a crankset, so they are normally found only on the outer rings.
In addition to the ramps and pins, modern chainrings designed for derailer use often have the teeth in the area adjacent to the ramps cut with shorter peaks than the other teeth, again to facilitate upshifts.
These features all help improve shifting, and make rider skill less important in shifting, but none of these features is actually essential to the functioning of any system.
See also Pins & Ramps.