A: If you are able to position yourself close to the preamp (regular instrument cable distance), there is no technical reason not to use the instrument input on that device. It is essentially a built-in direct box.
However, in many studios, where the players are in a different room or are positioned more than, say, 20-30 feet from the preamps, direct boxes should be used. (This is why direct boxes are used onstage; the mic preamp inputs are typically a long way from the stage.)
Another reason to use a direct box is for tonal coloration or the features it offers (depending on the direct box), such as phase invert, ground lift, signal splitting to an amplifier, adjustable input impedance, and more.