When to use subject pronouns

[jrhjazznblues]

I was lead to believe that you only use subject pronouns when the subject is not clear or to emphasize the subject, but I've noticed that my Chilean friend and his father always use subject pronouns, even yo, which I thought was never necessary. Is it more or less common in Spanish to use the subject pronouns when they are not necessary? Do you have to use the subject pronouns?

[AngelicaDeAlquezar]

It is true that pronouns are needed to avoid ambiguity in meaning, but they're also used for reasons of emphasis.

- Yo no sé nada de economía. = No sé nada de economía. -> Adding the "yo" makes the statement more emphatic about the fact that I know nothing about economics and I'm probably not interested in learning (or that the people whom I'm talking to do know much more about economics than I do).

- Tú nunca quieres hacer nada. = Nunca quieres hacer nada. -> If I add "tú", I want to make sure it's you who never wants to do anything.

- Nosotros no nos separamos por nada. = No nos separamos por nada. -> We want to make sure that it's clear we have a bond between us no one can break.