Why do you need to use the preposition 'a' when talking about a person?

When the direct object of a sentence is a person e.g. "ayer yo llamé a Juan", you must use the preposition 'a'. It is not necessary with inanimate objects, but any person/pet that you have a personal connection to must be referred to with the personal 'a'. For example you could refer to a pet with the personal 'a', "Sofia quiere a su perro" but not "Sofia quiere a su ordenador" as the computer is not a living being which she has personal relations with. There isn't a direct translation in English but the personal 'a' must be used even when forming questions, such as "A su madre necesita algo del mercado?". It is not used after the verb tener or hay, even with a direct object that is a person.