Information on a HealthcareService doesn’t tell you anything about the Practitioners who work there. A practitioner within a HealthcareService might be a custodian or receptionist or other individual with no healthcare expertise at all.
Practitioner.qualification tells you what education/certification has (but not necessarily what they’re doing)
PractitionerRole.specialty tells you what capacity a practitioner is using in their work in a particular location. Someone might be qualified as a surgical nurse, but they’re acting as a public health officer doing immunizations.