It signifies a "certain someone" or a "somebody," as opposed to "anyone at all".

or psychological texts (like Freud's discussions)? Grammatical drills ?

"Tristatur vestrum? Oret. Aequo animo est? Psallat."(James 5:13: "Is anyone among you suffering? Let him pray. Is anyone cheerful? Let him sing psalms.") Classical Grammar Usage: