I’ve mostly used it for finetuning the transient of some sounds, only ever went up to a parameter value of 3 or 4 on purpose, if at all. Nice on percussive sounds, I typically want the filter to be open whenever the pitch envelope has settled – a longer attack time might sound too “sweepy” on a drum, so I use a shorter attack time with a tiny bit of delay.
On less percussive-type stuff I find it nice for tweaking live, hitting FUNC while on the filter page switches the attack parameter with the delay. Can lead to some surprising grooves when that attack sweep shifts slightly out of sync with the not onset. You hear a slight, dull hint of the sound before it actually opens up.
For longer sustained notes it might also be nice to have the filter open at first – for let’s say a 8th note – and then have the delay close it in time with the groove. Like a step or something.