Wire Trellis, Tensioned or not?

I need to construct a pair of wire trellis for my dwarf apples and for my bramble berries.

First, what is the benefit of putting the wires under tension? I’ve seen some kits and some trellis that use a tension device (on a hook that sinks in the ground, usually) at each end, but what is the overall benefit vs just attaching the wire to a fastener?

Second, I’m thinking of running crosspieces on top of the bramble berry trellis and training a grape vine across them, either 3, 4, or 5 lines. I’m unsure if this is going to cause a shade problem for the brambles, though. The berry bed also has gooseberries and currants in it, but they pretty desperately need some shade here in 7a so I’m not real worried about them.

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Wires are put under tension to prevent excess sagging or out-bowing at the row center. The amount of untensioned sag would depend on the fruit/plant load. Dwarf apples and erect varieties of brambles (somewhat self-supporting) will not load the wires as much as grapes or trailing types of brambles.

Any tensioned trellis system is best with very firmly planted end posts. A top crosspiece will help stabilize the endposts so that tensioned wires will not draw the endposts inward. A grape/berry trellis combo should be fine if the grapes are kept a couple of feet above the berries.