Setting can be important because it can help drive the story forward. It can tell a reader or audience important things about culture and time period, which can further help relay the story and characterizations. Setting can also be used to help relay critical themes. It's actually very easy to determine setting in a Shakespeare play as he always describes the setting in the stage direction at the very top of the scene. Once you've determined the setting, in order to determine the importance of the setting, you can then ask yourself questions like, how does the setting develop the plot; how does it develop the character; how does it develop a theme? Below are a few to help get you started.
As we see from the stage directions given at the very top of Act 1, Scene 1, "Verona. A public place," the very first scene is set somewhere in public in Verona. More specifically, the setting is most likely out of doors, possibly even a town square, or a piazza, in which several streets of Verona would converge. We can surmise that the setting is out of doors near streets based on lines Prince Escalus gives in his speech accusing Lords Capulet and Montague of having wreaked havoc in Verona streets three times now:
Three civil brawls, bred of an airy word
By thee, Old Capulet, and Montague,
Have thrice disturb'd the quiet of our streets. (I.i.85-87)
Hence, since Prince Escalus literally refers to streets, we can assume that the public place is more specifically out of doors in Verona and possibly a piazza. The setting of the public Verona piazza is significant to help drive the story forward as well as relay the first most important theme. Shakespeare wants to show us just how much the Capulets' and Montagues' violent, uncontrolled emotions are causing unrest within the entire peaceful city of Verona, portraying the theme of the consequences of violent, uncontrolled emotions.
Based on the stage direction of Scene 2, we see that this next scene is more specifically described as "A Street." Since Paris is having a private conversation with Lord Capulet in which he is, once again, asking for permission to marry Juliet, it is most likely a quiet, more private street, possibly leading up to Capulet's home. The outdoor setting of the street is also significant because it presents opportunity to drive the plot forward. The conversation between Paris and Capulet ends with Capulet handing his servant a list of people to invite to his ball that evening. Since the servant is out of doors when he receives the list, it gives Romeo and Benvolio a chance to converse with the servant and conjure up plans to crash the ball, which is of course one of the most important moments in the play.