Properly used, commas can make all the difference in the meaning of a sentence. For example, compare these two sentences, one with commas and the other without:
"Taxes, which inhibit economic growth, should be reduced." Meaning: Because of the presence of the two commas bookending the restrictive clause (", which inhibit economic growth,"), the sentence as written means: "All taxes inhibit economic growth and therefore all should be reduced."
"Taxes that inhibit economic growth should be reduced." Meaning: Because of the deletion from the second example of the two commas included in the first example, which is reinforced by the substitution of "that" for "which" in the second example, the sentence as written means: "Some taxes inhibit economic growth and some don't; those that do should be reduced." In the second example, the entire meaning of the sentence turns on the presence of "that" instead of "which" to begin the restrictive clause, and the absence of the two commas setting offf the restrictive clause.
To illustrate the importance of the presence or absence of the commas, here is the second example without the change of "which" to "that" but with the two commas deleted:
"Taxes which inhibit economic growth should be reduced." Some writers disdain what they regard to be the fetish of changing "which" to "that" in the second example, because they believe that both sentences have the same meaning because of the presence of "which," regardless of the omission of commas to set off the restrictive clause.
In the second example shown above, the entire meaning of the sentence turns on the presence or absence of the commas. The failure to delete "which" and replace it with "that," which would have changed the meaning all by itself without the use of commas to set off the restrictive clause, leaves the sentence ambiguous. It doesn't clearly mean all taxes inhibit economic growth and therefore all should be reduced; and it doesn't clearly mean some taxes inhibit economic growth and some don't; those that do should be reduced. The reader is left to his own devices in attempting to interpret the sentence without any clear guideposts.
The insertion of commas to set off the restrictive clause, or the failure to insert them, governs whether the restrictive clause should begin with "which" or "that." The word "which" should not be used in this context unless it is preceded by a comma (or the clause that it begins is set off on both ends by commas). If commas are used to begin or to set off the restrictive clause, "which" should be used and not "that."