The sub-review process, when it works well, is arguably a reasonable one. To give the example of how this works from the perspective of the program committee of a conference I'm involved in:

The PC chairs assign papers to members of the PC. Those reviewers are ultimately responsible for the review quality and, a more frequent problem for the conference, ensuring the reviews are in on time. In principle, they can ask anyone to sub-review, but in practice, it usually goes to grad students, postdocs, or graduate alumni (and since we have a relatively light review load per member, we have many people who do all reviews themselves). The reviewers arguably know more about the expertise of their grad students and postdocs than the chairs doing the assignments do. Also unlike a journal, where editors might ask anyone with particular expertise, we both only assign reviews to PC members, and do assign them: PC members only get to state their preferences on what they would like to review. The sub-review process ideally lets reviewers ask people to do reviews who they know would be suited to a particular paper, but who might not be experienced enough to reasonably be on the PC itself with those responsibilities, and the chairs might not know much about. It then lets those reviewers look over the sub-reviewer's work directly, which might include mentoring them. While we do anonymous reviews, identities are visible to chairs, and one thing I've noticed when a chair, for example, is that grad student sub-reviewers often do excellent, thorough reviews, but also often lack the confidence to be sufficiently critical when writing about problems and weaknesses they identify, something that the reviewer can help with.

The review system (we use easychair) directly handles sub-reviewers, and our proceedings list all sub-reviewers (at least, those who actually submitted reviews). Good sub-reviewers can sometimes be reasonable candidates to ask to be on the PC the next year, and give a gentler, safer onramp: we're able to have a wider mix of junior and senior members when there are new postdocs (and I think in one case a grad student) who we already know do reliably good reviews and know our review process.