What is the origin of the Degrees, Decimal Minutes format?

It is used in marine navigation to make the maths simpler, as well as instrumentation like sextants.

Take the example of a navigator who wants to find the angle of a celestial body with the horizon, he'll first make coarse adjustments on the sextant to get to the nearest degree, then turn the vernier to make fine adjustments. The vernier is marked off in minutes so it can be easily read to 1/10th of a minute.

His next task will be to adjust for various errors by adding and subtracting minutes and tenths based on altitude, temperature, sextant inaccuracies and so on. Doing this in decimal degrees would be an excercise in frustration because 1 minute of arc isn't a nice round decimal, so there would be lots of accumulated error in the calculations after a while. If he used DMS, there would be lots of extraneous calculation for not much benefit - on a ship you could never accurately measure to within a second of arc. Nautical calculations only go to 1/10th of a minute, and you need to make at least three observations before you can be sure of your position.

So in this case at least, it makes sense to have GIS data in DDM if it's nautical in origin (or generated by someone with a nautical background), or if it's been derived from manual surveying.

I must admit, I was like you a while ago - I couldn't really see the point. But since I've been studying navigation, it does seem like a natural and simple way of expressing angles.

I'd guess it is because of a nautical reference too.

One of the "tricks" to rough navigation is that a Nautical Mile is (roughly) 1 minute of latitude, and also (roughly) 1 minute of longitude at the equator.