The significance of this monologue is in expressing the thought of Shakespeare, that the world is theatre, and all people are actors in it. 'World-theater' metaphor is the core of the whole Shakespeare's work. This monologue does not separate him with the First actor, but only makes them closer. It is based on the plot, which is far from being familiar to the English audience of XVII century, and can be supposed the most emotional of all tragedy. By asking 'What's Hecuba to him' Hamlet implies the whole story, taking place in the Danish kingdom, and this question makes Hamlet stand further away from the reader and from himself, making him the commentator of his own story. This monologue is the means of discovering the deep sense of tragedy - the metaphor 'theater-world' and to trace, how sophisticated is the transition from one reality into another.
The significance of Hamlet Hecuba speech is in being the principal Shakespeare's means of showing the implications and thoughts of the tragedy. The work is abundant in parallels and comparisons of the reality with the theater. Thus, the significance of the described speech in making this plot line evident can be subjected to no argument.