In Indian Classical Music the “Sa” or the tonic note is not fixed for a performance. This means the artist can choose any note as his “Sa” and the relative positions of the musical notes are more important. For example, I could D# as my tonic, but then the only thing is that the other notes should be fixed relative to the frequency of my Sa (which is D#).
It is to give the artist and the audience a reference to the tonic that the Sruti Box plays in the background. It is important for you to practice with the Sruti in the background, so that you will get a sense of the tonic note and the relative position of all other notes relative to the tonic.