In the Baroque period (and even today) operas began with the overture. It was an instrumental introduction to the opera. Eventually, these were separated from the opera and led to the creation/birth of the orchestra.
The orchestra initially served only as an accompaniment for the singers, then it began to serve also for the part of the opera that were instrumental, such as the overture, at the beginning, the interludes between the scenes and accompaniment for dance scenes.
Overture could be performed as stand-alone pieces of music, which led to the creation of the orchestra.