A rapid change in direction is how most water separators work.
Water does not know it should 'stay in' a horizontal run... water is carried by the air wherever it takes it.
True about water separators, but they can't get the water
vapor out of the air. The water vapor does travel with the air, but a separator only takes out the part that's condensing or on the verge of condensing when it hits the surface of the separator. If you were to run compressed air line from an indoor heated space to outside (colder), the water vapor would condense out of the air in the outside run and collect on the inside surface of the pipe. And once it's liquid water, gravity takes over and it runs to low spots, whether in the bottom of a horizontal run or down into drip legs. Like someone else already said, separators need relatively high velocity (rapid change in direction to use your words) to work well and a typical air system velocity is low.
What I would do on a run outside and back in is to pitch the pipe down in the direction that the air is traveling and put a drip leg with drain valve with a good size rise in the main just after it gets back inside. That way any condensed water vapor would run to the low spot and be drainable.
