It is always a good idea with questions like these to consider what would be lost if the play started at Act I scene 2. Think of how crucial the test of love is that Lear gives to his daughters at the beginning of the play, and the way that it sets the scene for what is to come. The love test of course invites only flattery rather than sincerity, and the response that Goneril and Regan give Lear certainly help to introduce one of the key themes, which is appearance vs. reality, because Cordelia is confused about how to flatter her father, placing her firmly in the camp of reality compared to the appearance of her sisters' devotion. She decides to "Love, and be silent," and thus gives the response "Nothing, my lord." Lear's inability to discern between appearance and reality is what causes him to treat his favoured daughter so harshly. Yet it is crucial for this very bleak play that we remember the catalyst for this tragedy is the real love of Cordelia and Lear's ironic blindness to the depths of her emotion.
While the opening scene of this play makes uncomfortable watching, it is therefore very important in setting the bleak and unyielding tone of this play, and is key in the way it introduces several themes that dominate the play throughout.