Police services can ask you to produce your certificate for scanning e.g. border check. Some transport companies may require a certificate to avoid unpleasant surprises upon arrival.
Belgium uses the CovidScanBE app for this purpose. Starting from August 13, CovidScanBE will also be used by event organisers.
For security and privacy reasons random QR code scanners will not be able to scan your QR code and access its data. This also goes for other entry tickets (e.g. movie tickets) that use a QR code.