If you have ever wondered why we use aluminium for fins in radiant floor heating here is a simple visual example:

This image is actually to show the benefits of fiberglass over other materials for windows (it conducts very little heat). But the image also shows that aluminium conducts three times more than metal.

Thus the usefulness of it in conducting heat from the pex tubes. It is also light, which is good in old brownstones with rickety old joists.