I'm curious about the same thing. With my Ada background, I see a completely sensible interpretation of MemoryHolder:

A Holder type is a managed (RAII/garbage collected) wrapper that has a reference to some unmanaged/primitive object. If that primitive object is some notion of a contiguous region of memory, then MemoryHolder is a perfectly good name for it.

A name like "Buffer" might not automatically tell the users that the type is managed, or that it wraps some underlying primitive type. That may or may not be relevant to the user, but it's not obviously always irrelevant, at least.