I assume SPIFFE is more useful to system to system authentication without the end user context - like how Netflix uses short-lived certificates to secure interactions between microservices (https://medium.facilelogin.com/short-lived-certificates-netf...) ?
Thats the primary motivation and main focus for SPIFFE. Providing service to service identity. However because its not breaking any of the standards its potentially applicable in other contexts. The SPIFFE SVID (the certificate standard) doesnt do anything wierd or different with TLS certs (which is actually a strength) it more sets out a way to use the current existing cert infrastructure to provide identity.