In Elizabethan England, as today, flowers carried symbolic meaning such that one could convey a message through the flowers given. While the original manuscripts do not identify which character receives which type of flower, a consensus has grown regarding who receives which flower and why. Rosemary and remembrance seem an appropriate request to make of Laertes. If Ophelia were pregnant by Hamlet or even feared it, keeping some rue for herself would make sense.
As is true in Hamlet's performed acts of madness, Ophelia's actual madness allows her to speak what cannot otherwise be said to powerful people, or, to quote Polonius' claim regarding Hamlet early in the play, "Though this be madness, yet there is method in 't" (2.2). Reserving daisies, since no one at the court warrants a token of innocence, intensifies the threat Ophelia's decline into madness holds for Claudius, already worried that Polonius's death will reflect poorly on him and incite Laertes's impulse to take revenge.
In most good productions, this scene is probably the most painful scene to watch. Ophelia has been mightily abused by everyone throughout the play. Her mad scene conveys all the pain and the sense of injustice she feels and is as wrenching as Hamlet's many soliloquies. As she gathers flowers and bestows them, she is also seemingly trying to perform a funeral rite for a father who has been killed and forgotten, leaving her alone in the court.
In the play, Hamlet's many soliloquies give us an opportunity to witness his grief and for him to express it to the audience. His growth of character is contained in those speeches to the audience. Ophelia, living a parallel tragedy of wrongful death and isolation, has no one to speak to and no one who will listen. All the content of what could be Ophelia's soliloquy comes out in the snatches of songs and the flowers she offers.