The primary use case here is obviously using your computer to control a modular synth set up. That could be as simple as just using your PC as a master clock to set a tempo. Or you could, for example, hook up a Moog DFAM and trigger different drum hits from within Live using a control voltage (CV) or play a bassline with your MIDI keyboard. There are also tools for modulation, like LFOs and envelope followers. All of which is to say, if you have a modular rig, controlling it and tweaking patches is now a whole lot easier. And this could be a particular huge boon in a live setting, where it could save you the hassle of searching for a tiny obscure knob or fighting with a tangle of patch cables.

You don't need a modular rig to use CV Tools, though obviously you won't be getting the most out of them. There's a Rotating Rhythm Generator plugin as part of the package, that lets you create "modular-style" beats in MIDI. You can then point that at one of Live's built in Drum Racks, a virtual instrument or even an external drum machine to create organic and evolving patterns.

CV Tools is a free download for anyone that owns Ableton Live 10 Suite or Live 10 Standard and Max for Live.