Power density doesn't always matter. There are applications where space is abundant, but regular maintenance is prohibitively expensive.
In my quick example of a remote monitoring station, it may cost $10,000+ to send a helicopter out to change the 12v car battery when it dies from exposure to extreme temperatures in 5 years or less. If something like this supercapacitor can last 20+ years without every be visited, it would be more cost effective.