In this way Shakespeare makes sure the reader understands both aspects of fate and choice which together forge the development, crisis, and resolution of the story.
If Romeo and Juliet are indeed "star-crossed lovers" where destiny (or fate) has played against them (as in Friar Larence's letter not being intercepted in time by Romeo), the feuding families of the Montagues and Capulets are still held responsible for the deaths of their children for having pursued longstanding hostilities rather than resolving them.
The prologue in this way sets the stage for the play's major themes (For example, "Learn to resolve conflict before it is too late") and establishes the play as a type of cautionary tale in retrospect:
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal lons of these two foes
A pair of star-crossed lovers take their life;
Whose mis-adventured piteous overthrows
Doth with their death bury their parents' strife.
The fearful passage of their death-marked love,
And the continuance of theri parents' rage,
Which but their children's end, naught could remove,
Is now the two hours' traffic of our stage;
The which if you with patient ears attend,
What here shall miss, our toil shall strive to mend.
The Prologue is important for a number of reasons. First, it is Shakespeare's use of a traditional theatrical convention, not surprising given that this play is one of his earlier, more immature works. (He is certainly more confident to dispense with convention and tradition in later works.) Second, it provides the "groundlings" (the uneducated members of the audience who might miss important plot details) with a summary of the play, lest they get lost in its details. Third, it establishes the dramatic irony that runs its way throughout the play. The audience, after witnessing the Prologue, is privy to the events that are to come. Therefore, the audience enjoys a godlike perspective, witnessing the characters as they experience the events that are destined to befall them. Therefore, the prologue reinforces the theme of fate - the protagonists are destined to die and there is little they can do to avoid this fate.
There are several purposes to this prologue. On the practical level, it gives the acting company time to change any sets and costumes needed. As a sign of the Capulets' position, it gives a sense of the substantial size of their household. Primarily, though, it is comic relief. This play is all desire and tragedy, and this is a bit of humor saying "Life goes on."