The answer is: when you need a lot of them. If you have outposts on large deposits you might need 100 mines to build a single outpost, and not only is that very time consuming, the amount of gears and circuits you need to lug along gets ridiculous. On marathon a mine is 10 gear, 5 circuit, and 20 plate, so instead of carrying 1 stack of 50 mines you would have to carry 5 stacks of gears, over a stack of circuits, and 10 stacks of plate. Assemblers are even more expensive (again on marathon), at 8 circuits, 15 gears, and 29 iron. Also it is extremely convenient to make an outpost building train that has all the basic materials and that is auto-reloaded at the base, requiring their automation.Why would you spend time setting up factories for, say, mining drills, when you can just hand-craft them on site, and you don't have to split your iron production again?
Also, you don't really need to split off iron again. I usually split off one line of iron+circuits and use that to automate all the necessities such as belts, miners, assemblers, etc, outputting in stack-limited chests, later replaced by logistic chests.
Totally agree, something to improve end-game resource gathering would be highly appreciated, especially if it is infinite and has challenging logistics, for example requiring sulphuric acid and a washing/separating system. Even requiring water would be interesting as it might require you to ship water around in trains.What happened to dirty mining? D: That's the one single feature I've wanted since the first few times I've played Factorio!