What purpose does the opening act of Hamlet serve?

The purpose of the opening act of Hamlet is that Shakespeare wanted to introduce the ghost before it could meet with Hamlet. That is accomplished by having Marcellus, Bernardo and Horatio all testifying that that ghost was not a person and he looked like the dead King Hamlet.

Furthermore, despite the fact that the ghost had an important message to Hamlet - he wanted to tell his son he was murdered by Claudius and he desired his son to take revenge - the introduction of the ghost on the opening act and his revelation to Hamlet only on Act 1, Scene 5 serve to build momentum with the audience so the revelation becomes even more of a shock both for Hamlet and the audience.