Tom Scott has a great video about the history of emoji. Basically, some companies in far east countries were encoding these icons in various proprietary codes for their messaging systems. The ecosystem became widespread and consistent enough that the unicode consortium saw it fit to include these emoji in the standard.

The snowman, on the other hand, is a weather symbol for snow, I assume. It appears alongside other symbols for meteorological phenomena, so I imagine was added around the same time and with similar reasoning: http://www.fileformat.info/info/unicode/block/miscellaneous_...