Almost every camera you can buy comes with some sort of light meter built-in so why would you use a hand-held meter?
There are plenty of reasons.
1. If you shoot with a studio strobe you really almost have no choice. You need a meter to register the short duration of the flash on your subject.
2. Hand-held meters are generally more accurate than the most sophisticated in-camera meters. Using a hand-held meter you have a chance to read the light striking the subject not reflecting from it. That makes a big difference.
3. It’s easier to measure individual light source’s impact on eventual exposure. If you want a strong main light (even if it’s natural light) you may still want a three-to-one or four-to-one ratio from your fill. The hand-held meter makes it possible to more easily determine that exposure.
4. It’s much harder to “fool” a handheld meter. A bride’s white dress won’t cause you to underexpose if you make your exposure with an incident meter. With most in-camera meters you’ll need to use exposure compensation to get an accurate reading in difficult situations.
5. Using a hand-held meter helps you slow down and take a more contemplative approach.