This sickle was probably originally used by Rivenese villagers for harvesting fruit or other food sources from the forested areas of Jungle Island. The Moiety began to use it as a weapon or all-purpose tool. The Moiety scout that darts Cho at the beginning of Riven uses the sickle to hammer a small dagger into the lever that controls the linking gate's bars.
According to Rand Miller during a demonstration of the sickle at Mysterium in 2006, it was also used as a hook to roll wahrks onto their backs to get at their soft underbellies. Given the respect (and fear) the Rivenese had for the wahrk, however, it's likely that Rand was referring to sunners, which the Rivenese were known to have hunted for food and clothing.