Friar Lawrence is Romeo’s advisor, but he is also important to the action of the play. He is the one who increases the suspense by marrying Romeo and Juliet and by allowing Juliet to fake her death.
Romeo clearly trusts Friar Lawrence. He is his advisor and supporter. He moves the action along by marrying Romeo and Juliet in secret. He does this because he thinks they will act anyway, and because he thinks that marrying the two might end the feud between their families.
In one respect I'll thy assistant be;
For this alliance may so happy prove
To turn your households’ rancour to pure love. (Act 2, Scene 3, p. 47)
By making this choice, Friar Lawrence unknowingly sets off a chain of events that does accomplish his goal, but also causes disaster for his young friend.
Friar Lawrence also agrees to help Juliet fake her death so she does not have to marry Paris. Romeo knows nothing about this plan, and Friar Lawrence’s inability to get the message to Romeo leads to both young people’s deaths.
I married them; …
You, to remove that siege of grief from her,
Betroth'd and would have married her perforce
To County Paris. (Act 5, Scene 3, p. 114)
Lawrence had a small part to play, but in realty his actions caused the suspense and drama. If he had not married them, and if he had not helped Juliet fake her death (or if he had gotten his message through), the tragedy would not have unfolded as it did.