The app works in Windows and Linux, and you can connect it via Wi-Fi or Bluetooth.
You can find instructions on how to configure it here.
Most newer computers come with a webcam, but if you're using an older system, you may not have one.
If you only need to use a webcam occasionally, it may not be worth putting the money into, so why not just use your smartphone?
It costs $2.99, but that's way less than you'd pay for another piece of hardware to do the same thing.