Not all cameras have built in light meters...but probably 99% of cameras made in the last 30 or 40 years do.
Any camera that has any sort of automatic mode, must have a light meter...otherwise, it wouldn't know what settings to use.
The light meter in a camera, measures the light 'reflectively'....it's a reflected light meter. That means that it's measuring the light that is reflecting off of the subject...and coming into the camera.
A hand held meter is very easy to use. Most can be use as reflective light meters, just like the camera. You hold the meter where the camera is, and take a reading toward the subject. Not a whole lot of point to this if your camera has a meter.
The real advantage is a hand held meter is that you can measure the incident light (if you have an incident meter). This means that you are measuring the light that is falling on the subject, rather than reflecting off of it. To do this, you go to the subject and point back toward the light and/or camera and take a reading. This is important because you are eliminating the reflectivity of the subject from the equation...which would otherwise affect your meter reading.
Also, when metering for flash/strobe lighting...you need a meter that is capable of that.