HPKE allows you to use private/public keypairs that are signed by a trusted authority, therefore enabling two parties that have never met prior to securely communicate. That is, as long as they trust the same authority.

Another major benefit of HPKE is a shared key schedule. Two parties can generate as many symmetric keys as they would like over a persistent conversation. This kind of provides ‘sub keys’, but within the context of a conversation, as they are not long lived.

GPG’s web of trust model is powerful but is limited in contexts where you have no prior trust. The world’s existing PKI structure, with a handful of root CAs, is already well established and trusted by nearly every device on the planet.