Transparency, depth, panning, separation, DAW total recall, pure analog signal mixing. Analog summing boxes have their uses beyond simply combining signals. The Summing circuit is a classic passive summing network. The same as found in countless mixers and consoles. The only difference is that instead of dedicated makeup amplifiers, the summing box has mic-level outputs which must be connected to mic preamps or a gain stage. Color and gain stage is made by your choice in your favorite gear.
For many people, the summing process occurs completely in-the-box and happens while bouncing down a final mix. You can choose to bounce it down in real time or expedite the process, but in either case, your DAW is crunching 1’s and 0’s to produce a stereo file. This is a digital summing. We do it analog.