If all frames are naturally broadcast in a LAN, what is the need for a broadcast address?
So that receiving devices can blindly throw away packets that aren't addressed to them or the broadcast address. Doing this test is much simpler than having to verify the CRC and tossing it up to the next layer that may toss it out itself.

A switch will also make use of the target address to only propagate packets to where it knows the destination address resides, reducing congestion.

The broadcast destination is used for stuff like DCHP where the sender doesn't know the target's MAC but still needs to send out the message.