Question:  

Please clarify when I need to buy the hub vs. buying just the switch?

Answer:     

If you just want to control a rocker switch or a button either with your mobile phone (via free app and Bluetooth), or set timers inside, the you don’t necessarily need the SwitchBot Hub.                   

If you want to make the SwitchBots work with Alexa, Google Assistant, IFTTT and more, or just for remote control, then the Hub is necessary as a Bluetooth to Wi-Fi gateway. 

Question:   

Can i control it using amazon echo or google home?
Answer:     

Yes. I use all my SwitchBots with Amazon Echo. I do not have Google Home, but the literature indicates that it will work with Google Home as well. You do need to purchase a SwitchBot Hub, however, it order to use with either Google or Amazon.

Question:    

Can I use this to remotely press my computer's power button to toggle it on or off?
Answer:      

Remotely: Yes.                    

1) Official and Simple Solution - SwitchBot Hub
If you have at least one "SwitchBot Hub" (the Hub) near SwitchBots (the Bots), which is a Bluetooth to Wi-Fi gateway and a rainbow color night light, then you could control your Bots from anywhere with Internet connection.

2) Bluetooth - 80 meters (260 foots)
SwitchBot itself is based on Bluetooth (BLE) with a (tested and verified) range of 80 meters (260 foots) in the open filed.

3) DIY - any Bluetooth to Wi-Fi gateway

SwitchBot has an open API available for makers to customize and integrate with Raspberry Pi and many others.