> I haven't been sucked into the Eurorack thing though

I'm the same. I love playing around with making electronic music on a hobby level and I find the idea and look of modular synths appealing - and I'm also a pushover for most retro things, especially those with cool knobs and blinkenlights. However, if I'm honest, I don't really enjoy creating music with modular or vintage analog "knobby" synths. I haven't ever bought a modular rig and my vintage analog synths are lovingly packed away with my numerous retro Amiga, Atari and Commodore computers where they wait to be enjoyed in limited doses on special occasions.

So, to address your implied question, IMHO I don't think people like us are somehow "missing" something deep and great in modular synths. To me, the essence of the modular appeal is three things: 1) tactile feedback that's responsively immediate, 2) a set of compatible 'lego block' components which can be combined in creative ways, and 3) An element of randomness from the combined interactions on analog components.

While modular rigs offer all three of those things, having those three together doesn't require analog hardware or a dedicated modular rig. I think I can get a very similar creative feeling and joy of discovery (plus a smidge of randomness) from the right combination of high-quality MIDI control surfaces and a well-chosen set of synth plug-ins running on a computer. To be sure, some MIDI control surfaces are crap and not all synth plug-ins enable creative experimentation deep enough and easy enough to 'scratch that itch'. But, then again, it's possible to assemble an ill-conceived modular rig out of poor quality components that also fails to inspire creativity. While finding that unique balance of factors sufficient to trigger creative serendipity isn't trivial with either analog hardware or digital MIDI + plug-ins, to me the advantages of digital in cost, size, speed, repeatability and flexibility win out.

I guess it's possible there's some other essential element which analog modular rigs provide that I'm missing out on but if so, I haven't been able to discover what it is.

Some people want to try new ideas that you can't buy ready made in a commercial synth or effects pedal. For example, trying unusual modulations, feedback loops, and signal dependent effects depth/parameters. Without a Eurorack you have to build a custom circuit to try it. With a Eurorack you can plug in some patch cables and quickly try it out. Think of a Eurorack synth module like a math symbol, you can assemble them to create an equation that produces sound. It's actually very much like an analog computer, where you can write an equation and implement it in hardware. It is literally possible to write equations and see what they sound like. You can do the same thing with something like VCV Rack though it's somewhat more difficult since to get physical controls you have to map midi controllers and may need a lot of midi controllers, which probably means manually labeling all the controls so you can tell what they do. In a Eurorack module each control, input, and output is already somewhat usefully labeled and defined by the module it is part of. So for some sound designers Eurorack systems are sophisticated sound design tools. You might use one to design a new kind of guitar effects pedal or to create a custom tweak on a specific sound in a movie soundtrack. Not that you can't just play with them without a coherent design in mind also, just to see what turns up.