Kunai originated in Japan and was first use as a gardening tool, but came to be used as a weapon in a variety of ways. Kunai are made from iron, so they were cheap to make and anybody could afford them. They were mainly used by workers of stone and masonry. They weren't used as a knife, but more of a crowbar, to pry, dig holes,and to smash wood and plaster. It wasn't sharp, only the tip, any sharpened tool would of easily been destroyed.
The kunai was eventually adapted for use as an improvised weapon by ninja. They could be used as a piton or grappling hook to scale rock wall and spires. Ninja at the time have adapted gardening tools as weapons in combat, and a kunai was a no brainer. They weren't made for throwing (you can still throw it and cause damage), but they were more for stabbing and thrusting. Due to popular belief, myths were told that ninja used kunai for throwing than stabbing. They are heavier than a shuirken but can be used in CQC (Close Quarters Combat) rather than a shuriken. They were primarily used as a knife, but could also be used as a make-shift spear by attaching one to the end of a stick. There were also various versions of the Kunai such as: long, wide, and even saw toothed.