Sometimes (not too often), один really just adds the same sense of indefiniteness as articles a/an/un/uno/una/un' do.
Один мой друг живёт в Канаде. (A friend of mine lives in Canada. /I
have a friend living in Canada./)
It's not about 'friend count', it just prevents from false conclusions about 'my only friend / FFL' which are possible in case один is omitted here (Мой друг живёт в Канаде).
As for your example, the word один is useful in a Russian translation, with the following word order:
Шопенгауэр написал на греческом биографию одного человека.
It keeps the original's (Schopenhauer wrote a biography in Greek) measure of indefiniteness (no ideas if it was the only biography he wrote), while the literal version with direct replacement of the article (Шопенгауэр написал одну биографию на греческом) has an extra emphasis on на греческом and leaves unnecessary questions: was he a prolific biography writer who wrote one of those in Greek / was it just one / could it be an autobiography?
It is true that some forms of Spanish and Italian indefinite article even coincide with the word meaning 'one' but in the function of article they just express indefiniteness, so unos doesn't mean 'ones', it means 'some' (items in plural). In English, a/an tend to mean 'one of the specified kind', not simply 'one'.