Scenes 4 and 5 of Act 1 are absolutely crucial to the play because they introduce one of the biggest dilemmas that haunts Hamlet for a large part of the action: is the ghost real or not?
The scenes, which present Hamlet going with Horatio and Marcellus to see the Ghost, present us with important information regarding the state of Denmark, where something is "rotten," according to Marcellus. The scenes also reveal the exact way King Hamlet died, according to the ghost.
These scenes also set Hamlet on his course of revenge and destruction. Based on the information that he is given about the death of his father here, Hamlet is honour-bound to revenge his father's death.
We even see the first element of procrastination in this play, as Hamlet plans to take on an "antic disposition" rather than acting directly. Note how Hamlet ends Act I scene 5:
The time is out of joint: O cursed spite,
That ever I was born to set it right.
Hamlet here expresses his sadness and deep sense of unfairness that it is he who must "set it right" and avenge his father, which foreshadows why he takes so long to accept his position in the play as avenger.