Well, that's certainly a creative way to do it. That code does set the ADC to free running mode in what seems to be 1MHz sampling. Every micro second it calls the interrupt handler which updates a variable. A timer is also set up and when it "ticks" it calls an interrupt handler which in turn takes the current value of that variable set by the ADC interrupt and uses it. So, the ADC is not running with the timer but instead both are being used sort of in concern to still basically do the same thing. Why would someone do that? I have no idea. This sketch wastes all sorts of time by constantly updating the ADC readings when there is no need to. It would have been better to directly couple the ADC readings to a timer but maybe the person who wrote that sketch didn't know how or otherwise was more comfortable with their approach. Still, the approach in that sketch will work so you can use it if you want. It'll be fine so long as you don't start to need more performance. The Due is 84MHz so it has power to spare for sketches like this.