Why did Voyager 1 need a map indicating its origin?

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Why was it necessary? If any species advanced enough to be retrieving alien artifacts drifting through space, also be able to calculate its trajectory backwards across millions or even billions of years? Based on the crafts velocity, and where stars were at specific points of time, and their effects on the crafts trajectory, shouldn't it be somewhat of an easy task for a team of alien astrophysicists to calculate where Voyager 1 set off from?

Not necessarily. The reason there is a map of its origins is at the time of its launch in the 1970s, there were theories of solar/interstellar winds and gravitational fields and currents existing in deep space. They were sparing no small detail to ensure that if the trajectory could not be calculated by some advanced space-faring race, at least with the data included, they would be able to use the map to calculate where the Earth is.