So why the broadcast address configurable by hand, what is the reason for it and what situations the automatically calculated address is not applicable?
The broadcast address is configurable by hand mostly because it is a good practice to allow the user to have more flexibility and freedom about what they want to configure.
Besides that, there are two reasons I could think of, that would need a manually configured broadcast address.
IP Directed Broadcasts:
These are broadcast packets directed to an external (remote) network. You can send a broadcast packet to that network's broadcast address and the router that is the network's gateway will transform it into a Layer 2 broadcast frame and forward it to all hosts in the network. However, these directed broadcasts are usually disabled by default, since they could be used in a malicious way, e.g., smurf attacks , DoS attacks, and more.
UDP Broadcast Packet Forwarding:
By default routers break the broadcast domain. However, since services like DHCP work by using broadcast packets, routers can be configured to forward broadcast packets to a specific address. This could be the DHCP server's address, another host on the network, or this can even be an IP Directed Broadcast address.
These are two cases where you'd need a manually configured broadcast address. The second case being for a router, though, but nonetheless, it works for a normal host depending on its configuration.
This is not applicable to all routers, example is given for Cisco routers.
You can read more about IP Directed Broadcasts and UDP Broadcast Packet Forwarding in Cisco's Broadcast Packet Handling Guide (pages 3-11).