According to Samson Raphael Hirsch, the Hebrew words for widow and orphan themselves indicate why these two categories need special protection.
Almanah, a widow, comes, Hirsch explains, from illem, “dumb”, and yatom, an orphan, from yatam, which is related to verbs which mean to mutilate or cut off.
The widow has “lost her mouth” with the death of her husband and has nobody to speak up for her any longer.
The orphan has “lost his hand” because his father is dead, and he no longer has anyone to depend on or to take him by the hand and lead him.
Our duty in both cases is lo te’annun, which Hirsch translates not as “do not oppress” but “do not let them feel their dependent position”. If, therefore, someone has no protector, society has to protect them.
Hence, in a broader sense, the Torah is laying down a pattern for any community’s social welfare agencies: those who cannot speak up for themselves must be given a voice, and those who have no-one to rely on must be given strength.