The purpose of a prologue is to introduce to the reader background information that is pertinent to understanding the story. A lot of times such background information can be introduced in the actual narrative of the story; however, sometimes doing so can slow down the story, or sometimes it's impossible to relay the background information without making use of a flashback in time. Therefore, adding a prologue is a useful way to introduce such information, as well as introduce characters and themes. The Prologue to Romeo and Juliet, like all prologues, certainly does introduce the pertinent background information that would have been especially difficult to relay in the story in its play format. However, in one way the Prologue in Romeo and Juliet diverges from traditional prologues is that it actually also tells us how the story will end.
The first thing we learn in the Prologue is that the story involves two noble families who have been engaged in a feud for many generations. Not only that, it seems that the families were peaceful for a while but have now begun fighting again, as we see from the line, "From ancient grudge break to new mutiny" (3). Without this background information it would be difficult to understand from just the dialogue that the fights we are seeing in the play stem from a very ancient feud that had been buried but has now resurfaced. Hence, just like traditional prologues, Shakespeare used his prologue to relay important background information.
However, unlike a typical prologue, Shakespeare actually also relays exactly how the story will end. He tells us that the deaths of the two lovers puts an end to the families' feud, as we see in the the lines:
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents' strife. (5-8)
Hence, while for the most part Shakespeare's Prologue contains pertinent background information, just like traditional prologues, unlike traditional prologues, it also tells the reader how the story will end.