You could ask a lawyer, I suppose. But the basic gist of this is "we don't know what we might need to do with this data in the future, so we put 'we can do anything with this data' into the ToS so that we know that if the need arises we won't find ourselves unable to do what we need to do with it." Any website that doesn't do this could find itself unable to implement new features or comply with new laws they didn't think of when crafting the original ToS.
At the very minimum a ToS needs to have some way to update and apply retroactively to old data, which ends up being "we can do anything with this data" with extra steps.