Why are fibers 'carded' before being spun into yarn?

Answer 1

When wool is sheared off a sheep it is tangled, full of bits of hay and manure, dirty, greasy, and smelly. Scouring (washing) takes care of most of the grease and smell and some of the dirt. Carding untangles the hairs of wool and helps to remove debris like bits of hay. When yarn is spun without carding or combing it comes out very lumpy and clumpy. Ozark Handspun Yarns are intentionally spun without carding. A link to their site below shows how yarn made from uncarded wool looks.

Answer 2

The fibres are carded to remove plant material and dirt, and to get the fibres all lying (running) the same way ready for spinning.