In both examples, the a preposition is preceding the indirect object of the sentences. The use of "le" and "les" reinforces that:
Le compraron la casa a mi padre.
Carmes les roba los collares a las personas viejas.
The text in bold in both cases refer to the same object. So
Le = a mi padre
Les = a las personas viejas
Dealing with indirect objects requires the use of the a preposition, as noted in the DLE:
- prep. Precede al complemento indirecto. Legó su fortuna a los pobres.
If you change the preposition for para, then you do not have an indirect object, but something that expresses the purpose of what is expressed by the verb, and note that then you do not need le/s:
Compraron la casa para mi padre.
Carmen roba los collares para las personas viejas.
Of course, you could say:
Carmen les roba los collares para las personas viejas.
But then les does not refer to las personas viejas, but to other object. You could also use de:
Carmen roba los collares de las personas viejas.
But thus you have a sentence with a direct object (in bold) and no indirect object.
So:
Carmen les roba los collares a las personas viejas. Carmen steals the necklaces from the elderly people.
Carmen roba los collares para las personas viejas. Carmen steals the necklaces for the elderly people.
Carmen roba los collares de las personas viejas. Carmen steals the elderly people's necklaces.