Here's why:
* Restrictive Clauses: These clauses are essential to the meaning of the sentence. They identify or define the noun they modify. No commas are needed around restrictive clauses.
* Example: "The book which I bought yesterday is really interesting." (The "which" clause tells us specifically *which* book we're talking about.)
* Non-Restrictive Clauses: These clauses provide extra information that isn't essential to the sentence's core meaning. They are set off by commas.
* Example: "The book, which I bought yesterday, is really interesting." (We already know which book we're talking about. The clause simply adds extra information.)
Important Note: If "which" introduces a restrictive clause, there's no comma. If it introduces a non-restrictive clause, there is a comma.