Round liner tattoo needles are generally used for lining purposes. For example, #12 gauge round liners are the standard for creating bold outlines like those seen in the American traditional tattoo style.
Smaller configurations are also useful for fine line tattoo work. This work is super detailed and often results in small but highly detailed tattoos.
Be aware that round liners are often not designed to hold huge amounts of ink, so shading with one could become a slow process!