These have many uses but are most often used for binding the end of a fiber braid with cotton binding thread. The size is not too important but the needle should be stiff and sturdy enough to be able to go through a fiber braid. In addition to being strong enough to penetrate a thick braid darning needles have large enough eyes to accommodate thick threads.
Other types of needles that are useful when making kumihimo braids:
Link to Bead spinner needles