The ability to (mis)use modules is a big deal (using a function generator or a sequencer as an oscillator using an oscillator as an lfo or lfo as an oscillator etc etc etc) the ability to re-wire everything all the time, there is no set signal path so a well put together modular could be thought of as several different synths (a drum synth, an fm synth a traditional subtractive synth an additive synth etc etc) and/or it can be all of those things at the same time. Depending how it’s planned you could have an entire music making machine in one box or you could have a very very capable and flexible synth.
The specific and interesting modules that exist nowhere else, stuff like Mutable instruments Marbles and Intellijel quadra+expander come to mind for me.
And of course above all the ability to fully customize your own personal synthesizer, one could build a traditional classic Moog style modular or a West coast Buchla style modular or a mutant drum machine or a monster effects rack or again all of the above in one.
I could say a lot more but you shouldn’t think of it as what can modular do that other hardware can’t and think of it more as what could YOU do with a set of specific modules.
I have owned a ridiculous amount of synths over the years and several of them have been billed as “a sound designers dream” and “like a modular in a box” etc but all of them regardless of how flexible their modulation matrix is never is quite like actual modular.
I first got into eurorack around 2013 , I still have some of my original modules and I’m still finding new and interesting ways to use them.
The fact is there is nothing equivalent to modular other than other modular, not in terms of sound (necessarily) but it terms of capabilities.