In theory at least, the actual heal will reach the target 0.5s faster than Cure II would. But that's a really rare/niche situation to get into and normally would've existed alongside other more glaring issue(s).
Another niche situation I've personally experienced was during P10S prog where I had somebody touched the poison puddle accidentally. I was playing as a SCH at that time and would rather save MP at the time for the sake of progging so I opted to spam Physick for the duration of the DoT. If I were a WHM at that time I would've used Cure I/Freecure for the same reason. The 400 MP cost & 1.5s cast time also mimics the need to slidecast just like our filler perfectly.
... I know at this point I'm reaching hard to think of a 'good reason why this button exist' lol.