Well "engineer" twisted pair helps in minimizing interference by in effect, balancing the two wires exposure to noise sources. If you run it over a fluorescent light for example, wire 1 is closer for the same distance as wire 2 because of the twist. It also maintains a consistent characteristic impedance for the length of the lines, but nobody worries much about that in automation. It is important for comms. It provides the best results with balanced line transceivers because noise shows up in common mode, that is on both wires with the same polarity. It also helps in single ended applications, but for slightly different reasons. Since the signal we want is almost always the difference, or voltage between the wires, balancing the noise to both wires keeps it from adding to the signal. Both types of shield are useful, braid is more expensive and bulkier, but provides better shielding if the coverage percentage is high. That is, a sparse braid or wrap may provide less shielding than foil. Some types of cable use both. Leaving the shield float may be much worse than no shield at all. This is quite common. If the shield can't drain induced currents to ground, it's really good at inducing noise into the signal wires. The best way in fact.
If ever you are driving in the country and see a set of telephone poles alongside the road take a good look at how the wires are run. Sometimes they run 4 wires with a right hand twist from pole to pole, top inside-top outside, bottom outside, bottom inside then back to top inside.
Other times they run them straight but every now and then have a double insulator or one with two vire slots then cross jumpers to switch position.
This is to balance out the induction from nearby power lines.
