Because VoiceOver ships built into every Apple device, it is the default screen reader experience for a huge number of iPhone and iPad users, which makes mobile Safari and native iOS app testing with VoiceOver essential rather than optional for any organization with a meaningful mobile audience.
VoiceOver has its own gesture-based navigation model on touchscreens, using swipes and taps that differ from desktop keyboard-based screen reader navigation, so testing a responsive website with VoiceOver on an actual iPhone, not just a desktop screen reader, catches a distinct set of mobile-specific issues.