Why do people use "of" in some phrases, like this one:
•"These are the contacts of my dad"
•"These are my dad's contacts"
I've seen sentences like the first-one so often.
English allows two ways of expressing possession. In some situations, one way may be preferred. In some situations you can't choose.
For example the so-called "partitive genitive"
A glass of water
cannot be changed to "water's glass"
On the other hand, "The book's cover" or "The cover of the book" are equally accepatable.
Generally "'s" is more common with animate noun's (Dad's tie) and "of" is more common with inanimate and abstract nouns, and especially inanimate nouns with more than one syllable (the end of the difficulty).
Although "These are my dad's contacts" would be the usual way to express it. "These are the contacts of my dad" is also correct.