The Ethernet MAC address FF:FF:FF:FF:FF:FF is reserved for broadcasts. 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.
The purpose is to inform the recipients that they should receive rather than ignore the message.
While Ethernet messages are "naturally broadcast", this is an implementation detail. Most messages are intended only for one recipient, and just happen to be receivable by the other addresses connected to the LAN.