Thank you for your question. A marja is responsible for deriving rulings based on sound methods from the scriptural sources when possible, or ascertaining the appropriate action that a Muslim should adopt in any given situation based on the principles of jurisprudence.
The correctness of a person's action is their own responsibility and for that reason, people in the time of the greater occultation are faced with two choices when they are unable to ascertain the rulings for themselves. They may take precaution in all matters they are not certain about (assuming they know when it is appropriate to take precaution) or they can refer to the rulings of a person who is qualified to ascertain those rulings to the best of his ability and act in accordance with them.
The main evidence for resorting to someone who is qualified, when a person themselves has no means by which to derive the rulings for themselves, is that this is the action of rational people. In the same light, the justification for corresponding with the rulings of the most qualified or most knowledgeable jurist (when the knowledge of who the most knowledgeable marja is, is possible) is that by following the most knowledgeable jurist, you can be more certain that your actions will be acceptable on the Day of Judgment.
May you always be successful.