Yes, it is necessary for fire code compliance. If you have a fire start in an electrical outlet in the wall, it could easily and quickly travel to the floor bays above, across the floor and follow a wiring/plumbing hole to the second floor space or attic. Code (minimum) requires fire blocking at wall top plates and every 10' lineally in the wall or behind it; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec002_par031.htm
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