Before the conductor walks on stage and the concert begins, the musicians need to be sure that their instruments are perfectly in tune with one another.
In order to do this, the concertmaster stands up and asks for quiet. Then the oboe plays a tuning note (an “A”) into an electronic tuner and musicians play that same note to hear if they are exactly in tune.
String players adjust their “A” string and then the other strings of their instruments if necessary. Wind players also adjust their instruments accordingly.
The reason the oboe plays the tuning note is not only tradition – it is also because the sound of the oboe is very penetrating.