Let's say it's 7:30 am. You start work at 8:00, and your bus leaves in 5 minutes. You make one last update to iCal on your laptop at home, adding a last-minute meeting scheduled for this afternoon. It's time to close your Macbook Pro and rush out the door to catch the bus.
If push is instant, then that last-minute meeting is pushed online right away, and you don't have to worry about forgetting where it's at or what time it's at: it will show up on your work computer and your iPhone.
If push happens every 15 minutes, and you didn't remember to manually sync it before you closed your computer and rushed to catch your bus, then you're out of luck: the meeting didn't sync, it won't show up on your iPhone or work computer.
Alternately, if you used a Mac Pro at home, it would mean you'd need to leave your computer on if you want it to sync by itself, which means it would be on all day, wasting electricity and money. Or else in either case, you'd have to get in the habit of manually syncing your updates from your laptop or desktop computer all the time, like in the good old days. Which isn't what most people consider to be "push".
I'm not saying it's the end of the world, but there are definitely a lot of circumstances like this one where instant push, as was originally advertised by Apple and is still advertised on at least some of the international version of the site, would be useful. If you're working on a computer and are about to leave, then you need to sync it yourself manually, instead of having it pushed online.
Good point.