I am Catholic and was asked why Catholics need the Mother Church as the guardian and interpreter of the Bible. So the church interprets the Bible for you, right?
How would one best respond to this?
That is what Roman Catholicism teaches. See: "Roman Catholic Quotes Regarding the Bible."
Since the teachings of Roman Catholicism cannot be fully justified from the Bible alone, the Roman Catholic church has taken the approach that it has the "right" to alter biblical teachings. Since some of its teachings contradict the Bible and are even internally self-contradictory (take the controversies over women as priests and priests being able to marry as examples), the leadership of the Roman Catholic church dissuades its people from doing their own investigations by telling them it is too hard for them to reach proper conclusions.