Relative clauses add information to a sentence, and can help make your writing more cohesive.
Defining realtive clauses are used to give information that identifies the person or thing which is being talked about.
The law that limits weekly working hours is called the Working Time Directive.
The relative clause is essential to identify the law - without it, we don't know which specific law is being discussed.
Non-defining relative clauses are used to give additional information.
My supervisor, who is really helpful, researches speech perception.
The relative clause gives us extra information about the supervisor, but without it, we still know who is being discussed.