Back in ancient times -- you know, before LEDs starting replacing every other kind of lighting -- the widely used PAR halogen bulb was known in part for its heavy glass lens. This lens was necessary to withstand the high pressure of the gas in the halogen bulb, and its seal also provided a "wet" rating for these bulbs so they could be used outdoors.
These PAR lamps were used in some cases for recessed lighting (although BR lamps are a better option for general lighting in recessed cans) as well as in outdoor spotlights and in track lights when someone wanted to highlight specific objects or areas.
Although they burn extremely hot, halogen bulbs offer high quality color rendering while using less energy than traditional incandescent bulbs and feature slightly whiter light than those incandescents.
As LEDs came to market, early versions didn't quite look like the bulbs they were replacing. Standard A19 bulbs, for instance, had large fins toward the back that acted as heat sinks. In a similar way, early PAR LEDs didn't have the look of PAR halogens that people preferred. And without the glass cover, many PAR LEDs were not rated for outdoor use.
But Philips has recently released a union of the old and new -- a PAR LED that looks like the PAR halogen lamps of yore, complete with a sealed glass cover that gives them a "wet" rating for outdoor use. So what's old is new, and while we may see the earlier designs sticking around for a while (given the long life of most LEDs), making the switch from halogen to LED now lets you retain the look you may already know and love.