It all depends on whether or not you need to monitor your sensor remotely and/or if you want to back up or sync your data with others on the SensorPush cloud.
The sensors use Bluetooth LE 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 SensorPush sensors remotely (e.g. away from the house), you need to use them in conjunction with a SensorPush WiFi Gateway, which relays the data of any in-range SensorPush sensors to the internet using your local WiFi or Ethernet to your network.
The other advantage is that is you are using the Gateway and multiple mobile devices are simultaneously monitoring the account, then your devices can stay configured and remain in sync. This means if an alert setting is adjusted in one location, it will also apply to the other mobile devices.