What was the Basilica's used for?

A "basilica" is a church, any church and the Word is Latin. Basilica derives from the Greek Βασιλική Στοά τρίκλιτη - Royal arcade threesided, a Roman public building. It had a rectangular design and inside the were colonnades usually two giving in the interior the sense of three divided spaces ending in an aspe oriented east to west. They were used as the churches of the new religion of Christianity which was tolerated after the decree of Constantine I the Great Emperor of Rome [313 AD/ CE] and was nominated official religion of the state by a decree issued by Theodosius I the Great [380 AD/ CE].

An image of a basilica you can see at the related link herebelow.