We list the eyebrow and lash comb together because they often sell as a unit. The lash comb is on one side and the eyebrow comb is on the other.
An eyebrow comb (sometimes referred to as an eyebrow brush) gives your eyebrows shape and definition, and puts any rogue eyebrow hairs into place. Also if you use an eyebrow pencil, the brush will blend in the color.
If you wear mascara, you need an eyelash comb. Mascara can clump, and an eyelash comb will remove those clumps. And as a side benefit, by brushing your lashes up and away from your eyelid, you'll give them a little curl.