The advantage of this method (which sourd'oh properly identified as a double boiler) is that it
- is slow
- has the food accessible in the open.
It is used for the sauces sourd'oh mentioned not just because of the limiting effect, because this would be too hot for most foods with which it is used. It is used because certain foods have to be worked in a certain temperature range. Egg yolks start binding at maybe 50 celsius, and are overcooked at 83-ish. You can't beat a hollandaise on the counter. But put it in a pan on the hob, and the part on the bottom will overheat. So the double boiler is much better. Besides, you want a round bowl for proper beating, and heating a round bowl on a planar hob is a pain.
Every time you have to stir/beat something while heated, the double boiler not only has an advantage over the microwave, you cannot even use the microwave for that.
The advantage of the microwave above the double boiler is, in turn, its speed. If you want to heat stuff which doesn't react badly to quick heating and doesn't have to be stirred or beaten, it is less fuss and more speed.
But if you have a reasonably quick hob, the sole advantage of the microwave above it is that you can heat stuff in the container you want to use for serving. For example, you can heat a porcelain mug of milk in the microwave, while you have to use a metal pan for the hob and refill the milk, giving you one more item to wash. However, the milk from the microwave will have hot and cold zones (happens to me always, even though the mw's I used had a turntable), while the hob heated milk will be more evenly heated. Conclusion: a hob plus a dishwasher reduce the advantages of a microwave to zero, while the double boiler is a niche tool which can never be replaced by a microwave.