Well the better ones have a white cone so that they can take reflected and incident light readings.
But an obvious practical advantage is where the camera is on a tripod and you want to take two exposures -one for the sky - one for the foreground. As it is now you'd need to take both readings before you put the camera on the tripod (to avoid moving the camera between readings) and either remember them or jot them down. Sometimes the light can change appreciably while you're doing that, but it's easy peasy to point that meter anywhere you like to obtain a reading.
Truthfully, in - camera meters are pretty good these days in most situations, and I wouldn't advise getting one if you're not going to use it, but they can be invaluable at times and learning how to use one will probably teach you more about exposure than anything else.
Spot meters in cameras are usually around a 5 degree spot, whereas better handheld meters have a 1 degree spot so it's possible to take very accurate readings from a particular tone in your scene.