The app is privacy-friendly because your personal details are not shared with event organisers or verifiers. They only see a unique visitor ID that cannot be traced back to you. Only after your explicit approval personal details, like your name, are shared with a verifier.
Event organisers find the app useful because it makes paper attendance sheets no longer necessary. These attendance sheets are difficult to use: people are asked details like name, time and signatures. All this data is usually visible for everyone attending the event. The logical consequence is the event organiser has to comply with GDPR rules. With the SSA app this is not necessary.