Incident Light Meters measure light that is directly hitting the subject as opposed to the on board Camera Metering which is measuring the reflected light.
This generally means, light has to be fairly stable or controlled to retain correct exposure long enough when using an incident Light Meter.
A good example of using an incident light meter outside in bright daylight, would be capturing the Bride and Groom at a wedding.
To explain this further, if you were to take a reflected reading with your camera off the Bride, who lets say for the purpose of this discussion, is fairly light skinned with a bright white dress.
The Groom on the other hand is a fairly dark individual with a dark suit on. Now, if you hover the camera over the Groom, you will find that the Groom is under exposed!
This is something very commonly observed when looking at wedding photos taken by family members where in bright daylight, white areas are overly bright and blacks areas are just too dark.
Things change when you use an Incident Light Meter.
You hold the Light Meter with the white Invercone facing towards the Camera in front of your Subject IE Bride and Groom.
The light hitting the cone is broken up and hits the reflector at the back of the cone which then measures the light and provides the correct exposure.
The Light meter can also allow for manually altering the aperture allowing for better control over depth of field whilst at the same time, altering the ISO and shutter settings accordingly to maintain the perfect exposure gained from the light hitting the reflector. As a result, both the Bride and Groom remain perfectly exposed.
The downside to using a Light Meter outdoors is that you do need to keep an eye on the light and take periodic meter readings to ensure correct exposure.
So, the biggest benefit of using a light meter outdoors is that you will always be able to gain the correct exposure of your subjects and not exposure gained from reflected light which may not always be just from your subject alone.