In other words, the idea was that if the practice of Katatagae for travelling to one place to another except one's home should be more lightly treated than that of involving one's home, it was convenient to assume that a place which, in fact, was not one's home, would be 'one's home'.
つまり、自宅より出先の方が軽くて済むのであれば、本来の自宅以外の場所を「自宅」ということにすれば良いという考えである。