What is the difference between wet/dry and reverberant/direct? Why are there separate controls for these?
There is a very important difference between wet/dry and reverberant/direct. When you are in a real acoustic space the sound source emits direct energy and the room reflects reverberant energy, and you hear a balance of the two. Moving closer to the sound source will yield more direct sound, going back further in the room means more reverb and less direct sound. Also, the direct sound in a real space has a specific direction and level matching your orientation and distance inside the room. In many reverb plugins, wet simply means the sound processed by the reverb and dry the sound that is bypassed without processing and you get the mix of the two. But if you think about it further, this will likely not deliver a realistic result since the unprocessed dry sound may not have the matching panning and gain as if you were in the room, even if you wanted these to be matched.
So in INSPIRATA, Wet/Dry stands for processed/unprocessed, whereas reverberant/direct means how much energy is coming from the direct sound compared to the reverberation. You can change them both independently. Normally, when you use INSPIRATA, you would want to set 100% Wet to let INSPIRATA handle the direct sound properly for you, and adjust the level of Direct versus Reverberant or simply move your listener position to a closer place if you want to make the sound closer or more distant. This way you can ensure that your direct sound is automatically panned and leveled correctly in the acoustic space, in surround. If you use the Wet/Dry slider instead, your 'direct sound' will be most likely very different from what it would sound like in the real space. Moreover, you may end up with two direct sounds conflicting with each other: one coming from the unprocessed signal chain and another from the sampled reverb in INSPIRATA.
When you place your dry sound in a completely damped room with zero reverberation, you still have a distance and dry panning information corresponding to your virtual source and listening position, and this is exactly what INSPIRATA is taking care of by processing the dry sound. So you need dry sound processing in order to get your direct sound coming from the right angle and level in an acoustic space which INSPIRATA automatically does for you. This is bypassed if you decrease the Wet/Dry from 100%.
When upmixing to 5.1 or using INSPIRATA as an insert effect, it is easy and obvious to get the right sound, but when INSPIRATA is used as a Send Effect on an Aux bus, you may need a bit more attention to set this up properly, as one might be accustomed to adjust the wet/dry slider. It is worth considering using 100% wet and adjusting the direct/reverberant slider instead.