It all depends on whether or not you need to monitor your Meter remotely and/or if you want to back up or sync your data with others on the SwitchBot cloud.
You can use SwitchBot Meter alone. You can either check the readings from the LED screen directly, just like the traditional thermometer and hygrometer.
Optionally, you can use the SwitchBot App to pair your mobile phone with the Meter. This unlocks more possibilities like getting the last 36 days of historical data (via Bluetooth), getting alert notifications, calibrating the Meter, etc.
The Meters use BLE to communicate, so on their own they are limited to Bluetooth range (e.g. around the house - like a Bluetooth speaker).
If you would like to monitor SwitchBot Meters remotely (e.g. away from the house), you need to use them in conjunction with a SwitchBot Hub (Plus), which relays the data of any in-range SwitchBot Meters to the internet using your local Wi-Fi.
If you have a SwitchBot Hub (Plus), this will connect your Meter to our cloud service. It means your data is always being uploaded and backed up automatically and boosts the speed when you check historical data (via the Internet). The Meter also works with Alexa, Google Assistant, IFTTT in this case.
If you like, and you have a SwitchBot Hub Plus/Mini near your Meter, you can make all SwitchBot products work together, e.g. when the temperature is too hot, turn on the air conditioner with the Hub Plus and Hub Mini(via infrared), turn on a Fan with the Bot (by mechanically pushing a button).
* Bluetooth alert requires your iPhone's location permission under iOS's rule.
Bluetooth alert for Android is under beta testing now thus a Hub (Plus) is needed.
**With the "Scene" feature, you can set certain conditions to trigger actions of SwitchBot devices or IR appliances.