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Nonrestrictive relative clauses are parenthetic, as are similar clauses introduced by conjunctions indicating time or place. Commas are therefore needed. A nonrestrictive clause is one that does
William Strunk JR. and E.B. White • The Elements of Style, Fourth Edition

Use a dash only when a more common mark of punctuation seems inadequate.
William Strunk JR. and E.B. White • The Elements of Style, Fourth Edition
"The airline pilot who announces that he is presently anticipating experiencing considerable precipitation wouldn’t think of saying it may rain. The sentence is too simple— there must be something wrong with it. But the secret of good writing is to strip every sentence to its cleanest components."
William Zinsser • On Writing Well: The Classic Guide to Writing Nonfiction
on writing well
Regina Casaleggio • 1 card
A comma is preferable when the clauses are very short and alike in form, or when the tone of the sentence is easy and conversational.