They are an essential part of the meaning of a sentence and therefore they cannot be left out.
Eg: It came from the man who held the reins.

In defining relative clauses we use a general pronoun "that" which is independent of the personal of non-personal character of the antecedent and also of the function of the pronoun in the relative clause:
Eg: It is safer to accept any chance that offers itself…
…out for anything in the whole cycle of labour that was required in the fair.
In defining relative clauses we use also "who". It is preferred to "that" when it is subject and when the antecedent is personal. But "that" is preferred to "who(m)" when it is object, in part perhaps to avoid the "who/whom" choice. When the verb in the relative clause is "be", the complement pronoun must be "that" or "zero". Provided the relative pronoun is not the subject of the relative clause. A further option exists in relative clause structure of having no relative pronoun at all. It is the clause with "zero" relative pronoun.
Eg: Nobody appeared, and he heard the person!!! retreat among the bushes. /that/
Some people!!! look upon marriage as a short cut that way,… /who/
George, the elder,!!! exhibited an ebony-tipped nose… /that, who/
Just as "that" and "zero" are available when the relative pronoun is dominated by a preposition, so they can be used when the relative pronoun is part of a place, time, or cause adjunct. With place adjuncts, the preposition must usually be expressed. But with the time adjuncts, omission of the preposition is usual whether the pronoun is "that" or "zero".
More frequently and more formally the pronoun "which" is used and the preposition precedes the pronoun:
Eg: The dog took no notice,for he had arrived at an age at which all superfluous barking was. He had just reached the time of life at which "young" is ceasing to be the prefix of "man"
We are brought on to a day in February, on which was held the yearly statute…

With manner adjuncts, it would not be abnormal to find "which" with a preposition in a more formal style.
Eg: … in a piping note of more treble quality than that in which the exclamation usually
embodies itself…
The main forms of pronouns used:

PERSON THING
SUBJECT Who (that) That (which)
OBJECT --- (that) --- (that)


Non-defining relative clauses

Non-defining relative clauses add extra information of secondary importance, and can be left out of a sentence. Non-defining relative clauses are separated by 2 commas and in speech we also separate them by pauses. Non-defining relative clauses are mainly found in written English, where sentences are carefully constructed.