When to use reflexive verbs?

Question

The title says most of it, but I'll elaborate. I understand reflexive verbs and I understand what they mean when I read them in context, but it's very unclear to me when I should use them. For example:

Desde dentro de la chimenea cegada se podían oír voces. (I'm reading Harry Potter 4)

Why use poderse here? I've seen plenty of identical contexts where they didn't use the reflexive pronoun and after all, the ability to do something is always reflexive to some extent or another so why not just always make poder reflexive? I can't seem to discern the pattern.

Thanks for the help.

Grant

Update: The above is not strictly reflexive, but is actually a passive use of the pronoun se. For more information see here: http://www.spanishdict.com/topics/show/87

A vote for filling out your profile, Grant. I hope you get 2 more at least and vote for them as well! - Jubilado, Apr 3, 2016

I gave him one. - Daniela2041, Apr 4, 2016

Answer 1

In this case the reflexive is used to convey the passive. This happens often in Spanish.

"From inside the blocked chimney voices could be heard. "

Answer 2

That's an impersonal passive "se", meaning "voices could be heard".

Thanks for your response! To clarify for those who see this, I don't think it would be impersonal, just passive after reading. - grantcurell, Apr 3, 2016