Why is it used "c'etait" with a meaning of passé composé ? I.e. "C'etait une belle soirée " Thanks!

Because the "soirée" is finished and they are probably saying good by to their host; The meaning is not "passé composé" , since the "soirée" is over:

Thank you massy but in my opinion "c'etait" is conjugated " à l'imparfait" and the meaning does not correspond to that tense theoretically. Thanks again!

Bonjour Máximo,

The verb être does not have distinct imperfect vs passé composé meanings like many other verbs do - ça a été and c'était are both normally equivalent to "was/were."

This is also the case for avoir, which is normally just "had" in English.

In comparison, action verbs like aller do have different translations: je suis allé - I went vs j'allais - I was going.

Muchas gracias Laura!!
Merci bien!!

Because it was lasted for a while, and was incomplete.

Because the phrase is descriptive, thus imperfect.