Generally, the wider the tire the more tire contact you get with the track. Usually, the more tire contact the more grip you get.
However, depending on your chassis design you may run better with narrower tires on certain corners. For example, people run a narrow left rear (compared to the right rear) because of loading in the turns through the rear axle, as well as for stagger choice options.
If loading is the same, a narrow tire will heat up faster (and sometimes more) than a wider tire. That heat effects grip too - generally more grip with more heat... until you overheat it (and lose grip).
