it's a local variable, and local variables do not get initialized in C.

But the initialization of the variable is right after the memset:

    address.sin_family = AF_INET;
    address.sin_addr.s_addr = ((struct in_addr *)(host->h_addr_list[0]))->s_addr;
    address.sin_port = htons(port);

Why is it useful the put 0 before setting those values?

It's good practice in general. There might be other fields that were not explicitly set, or the struct may expand in future versions, or you have union fields/bitfields (e.g. you explicitly set the low order bits of some union field to some value, thinking the high order bits are all zeroes, but because you neglected to zero them out first, they're filled with garbage bits. when you read the union field as a whole you'll get incorrect values).

Thanks for the explanation!