Umm, because only the Arduino has the built-in capacity to handle analogue input? It's quite common to use the two together because of this. The other option

Umm, because only the Arduino has the built-in capacity to handle analogue input? It's quite common to use the two together because of this. The other option is of course to just use an ADC chip with your RPi, but since most of us who bought RPi's already have Arduino(s) lying around the house, and since they're so easy to interface with Linux devices like the RPi, it's arguably the easier and more convenient option. ADDENDUM: The responsive design here is actually quite nice. If you want to be on mobile (like most home automation would), it's also the only workable alternative to the arduous task of creating apps for Android and iOS and any other platform you want to support.