From the beginning of the play to the point in which hamlet has his monologue in which he says "To be or not to be" the audience never sees hamlets true emotion. It isn't until this point in the play that hamlet lets his guard down and shows his true thoughts and feelings. It's the game changer for the audience, the other actors and hamlet himself. Because Hamlet portrays his feelings outwardly for the first time the audience is finally able to blatantly pick up on how he feels. Not only that, but this one line is the beginning of hamlets entire plot that he is scheming.
This is by far the most significant quote of the play. It is not only the turning point but one of the largest pieces of insight the audience or reader gets throughout the entire play. It is the one part of the play where the tone goes from agner to sorrow. It shows Hamlets inner feelings and gives the audience a perspective as two why he is feeling so deeply towards everything. He questions his entire life with this one line and is able to open up the doors of the play to every moment that Hamlet is scheming.