Because you can switch the GPIO pin off when you don't need the sensor and only on very briefly when you need it.
But in order to drive a sensor you need to check its current needs, since the GPIOs can only source up to 20mA (with this figure in mind you can check if your BME280 or any other 3.3V sensor would fit the bill yourself).
But what sensor do you have that "requires at least 3.6V"? The BME280 doesn't.
Because the LiPo or USB or Vin pin will still feed the 3.3V regulator and that's not powered down, otherwise the Electron wouldn't be able to wake again 