well google's (very)long term goal is to put ads into your homes after they take it over. Ie: If you run out of milk, google will remind you then you can click and have it delivered. If google detect you running heat all the time at a high setting via thermostat, they may display ads related to items for cold temperature/location, or vacations to warmer places etc.. I am sure they have better use cases.
But i agree google way overpaid for nest, it's a running joke. You buyout a company mainly for the below reasons:
1) talent - negligible, what it had like 10 guys?
2) marketshare - as drownpruf said, nest has a fraction and there is little ecosystem/lock in for those existing customers, again negligible.
3) remove competition - see above, nonfactor.
4) technology(and patents) - this is really not rocket science there is nothing proprietary, it's just wireless thermostat/co2 detector etc.. All those posters going on and on about their tech. It's laughable. Google could done it themselves in a few months - just slap a thermostat with bluetooth, comeup with an api, and an android gui you done. Wtf....
I really don't understand what google sees in nest that's worth even 1billion, let alone 3billion. This is nuts! There must be something else going on? They have a lot of smart people, cant be that stupid?