Dear Miku,
Relative clauses are used to give extra information about something without starting another sentence. They are one kind of complex sentence (as opposed to short simple sentences). When writing at university most kinds of writing require complex sentences, so learning about how to form and punctuate relative clauses can be very useful for QM students.
BTW They are also called "adjectival clauses" and there is a definition under that title in the Grammar Glossary, if you want more examples.