Halliday & Matthiessen (2014: 469):
In such cases, there may be an explicit Subject in the dependent clause, as in
||| It’s a much bigger house, || for the children to have their own rooms. |||
But in most cases of non-finite elaboration, the Subject is left implicit, to be presupposed from the primary clause; and it is often difficult to identify it exactly – e.g. in the hairy coat holds a layer of air close to the skin, insulating the body against changes in the outside temperature, is it the hairy coat which insulates the body, or is it the holding of a layer of air close to the skin? The question is really irrelevant; it is precisely the function of the non-finite to make it unnecessary to decide: the absence of the subject decreases the arguability of the clause.