Why do leftists use "right wing people do it too" as a defense?

Say I ask a question about leftists doing something. Then I get answers along the lines of "Right wingers do it too" as if that defends or excuse leftists doing it.

Is it their way of telling us anyone who thinks leftists are better than right wingers are wrong?
Perhaps they mean to tell us that when a right winger use a "leftists do it too" we should consider that a great justification of of right wingers doing the same they claim leftists do?

Why do leftists use "right wing people do it too" as a defense?
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