If the subject of a sentence is normally left implicit, what does adding it to a sentence mean?

Question

In Italian, the subject of a sentence is normally implicit.

Ho incontrato Luigi e mi ha detto di salutarti.

Siamo andati a Roma per il weekend.

Sono andate via senza dire niente; non so a che ora tornino.

This is not source of confusion, since a verb is normally declined differently for each person, plural or singular.

Does making the subject explicit change the meaning of the sentence, or does the sentence have the same meaning? Is it used as emphasis?

Io ho incontrato Luigi e mi ha detto di salutarti.

Noi siamo andati a Roma per il weekend.

Answer 1

Io ho incontrato Luigi is not ungrammatical, but is used only to give special emphasis to the fact that I met Luigi. So

Io ho incontrato Luigi e Marcella ha incontrato Laura

or

Io ho incontrato Luigi e gliene ho dette quattro

(meaning the no one else did). Otherwise, the pronoun is not used. A different situation is for the third person pronoun that's used more frequently.

Luigi scese le scale e andò verso Marcella. Egli l'amava, ma non gliel'aveva mai rivelato.

Here egli is necessary to disambiguate the meaning (in spoken language and more and more also in written language it would be lui). Note the difference:

Scesi le scale e andai verso Marcella. L'amavo, ma non gliel'avevo mai rivelato.

No io, which would sound clumsy.