Why do we need SocketOptions.SO_BROADCAST to enable broadcast?

If we want to broadcast information from a socket, we need to enable SocketOptions.SO_BROADCAST. However, I don't understand why that is necessary.

My understanding is we set the packet with a broadcast address, just the same way as set a unicast address. Then we just need to send it through a regular socket. If its a UDP socket, then a UDP header will be added to that packet, and then an IP header containing the receiver's IP address (in this case is the broadcast address in the form of 192.168.255.255), and then a MAC address (FF:FF:FF:FF) is added.

I think the router will get the packet and perform the broadcast. I don't understand why we need to set the socket attribute to SO_BROADCAST.