Ever look at detailed drawings of where a drain plug is? There's always a lip below which the oil cannot drain from the drain plug. An oil extractor can get down in that space before the lip and suck the oil out.
It's just a whole lot easier, particularly if the oil filter is located accessible from the top of the engine, to extract the oil rather than use the drain plug. I did a test on all my engines when I first got the extractor. I'd extract the oil and then open the drain plug. In most case, NOTHING came out. This was especially the case on my (then) JD 4010. No getting hot (near 200 F) oil on my hands (or hopefully nitrile gloved hand).
Ralph