1) Tsunami early warning system. The difference of 30 seconds might be enough to save a life. 5-10 minutes might save many more?

2) Stabilize machinery, close windows, blinds, turn off gas, start emergency generator in hospital. I'm sure the financial people would be able to sell stocks with earlier notice than others!

One reason is that I've noticed sometimes it takes me a few seconds to be able to tell if it's genuinely an earthquake or if something else is causing a vibration, and sometimes it actually takes a few seconds for it to become noticeable... then a few seconds on top of that for me to figure out how to react. Any kind of confirmation notification at that point would be extremely helpful to snap me into action and avoid needless delays.

Another reason is that, well, I might be asleep, and ideally I'd prefer to wake up with a notification, rather than (say) with something falling on my head...

Shinkansens apply full breaks automatically if dangerous earthquake is detected (there are about 50 on the way just between Osaka and Tokyo). Thanks to this system many trainsets will already be stationary when the seismic waves hit them.

The real-time notifications to cell phones in Japan are faster than the actual seismic waves, so if it's a decently sized earthquake a little ways away, it gives you a 5-15 second head start