The knock sensor is not all that magical. It's just a microphone that listens for a specific frequency. You can have pinging on one piston and the knock sensor may not detect it. Also, on an 8yr old car, they may be carbon deposits on the piston, that can cause auto-ignition, because they act like a glow plug. Appropriate fuel is an insurance against all this, running with inferior grade petrol is just like poking an angry bear with a stick.
Sure, it makes no difference for a N/A forfour in good working order to use 98ron, but on a brab it makes all the difference. Especially on a warm day, where you give it some!
And the tank is 40 something (47liters) so if you refill when the last bar shows up, you have gone through ~35L, that's ~7L/100kms, and 39MPG. If you go until the last bar is gone and starts showing in liters for reserve, than it's even worse.