For modular in general - the custom routing and voicing (pick your own oscillators and filters; add ring mod, wave folding, or other wave shaping to taste).
Can you do a lot of that in fixed architecture synths? Sure, and it can be hard to add something like a wavefolder to a fixed architecture synth that does not have one already. Some fixed architecture synths give you quite a lot, though (like the Novation Summit - especially with the added front panel controls over the peak). OTOH, it’s not a carryon sized synth.
Eurorack specifically? The largest market format. That doesn’t necessarily mean the best, but the dominant tech will draw makers because it is the dominant tech.
I saw Euro, 4u, 5u, and AE modular well represented at Knobcon. With enough motivation you can make any format work with any other, too.