If we were physically interacting with our users (i.e. we ran a shop or a community centre) we'd be using thousands of signals to determine who needed help, who was afraid, who was ready to purchase more and who was making trouble.
As developers we try to cultivate online social environments, socially-engaged shops, games which envelop the user and collaborative business tools but with absolutely none of these emotional cues. That's hard.
Imagine trying to design an amusement park if all you had was an anonymous ping each time someone went on a ride. The value to all of us (and to the users) in these new wave of analytics is to take us closer to the user and let us feel what they feel and service them where they need to be serviced.
Asking why one should do that is about the same as asking why you'd need to watch people queuing for the rides in an amusement park in order to improve the queues. Because if you don't you won't know what the user feels, wants or needs.
(Side note: what mixpanel is doing is incredible and they really are a pleasure to use. There is a Zen quality to their product and the way it gives you great power from great simplicity (although a custom dash would be great, ty! ))