why do we need Nest thermostat?

It is a little fishy, but..
I personally think most people who get one either like how it looks, that they can monitor/control it via app, or have been sold on the idea that having the "Nest" installed will somehow save them money on their electricity bill.
Looks are subjective, but it does look kind of cool when compared to a traditional T-stat.
It is quite easy to set the temp/operate, plus the menu/interface is somewhat intuitive (for me as a Ref. & A/c Tech.) to do advanced system tests or set-up, without needing the manual.
I have never used the app so I couldn't tell you how well the monitoring/control works, but EVERY mfg of thermostats now offers some form of wifi/app control for a few of their products. The nest, used manually, offers no real benefits but could save "some" money "if" set-up/programmed properly. I don't own one, or really care to. When I get my own house I will most likely do some for of automation on my own.
Love the Audio/Control set-up by the way!