Why was the switch statement designed to need a break?

Many answers seem to focus on the ability to fall through as the reason for requiring the break statement.

I believe it was simply a mistake, due largely because when C was designed there was not nearly as much experience with how these constructs would be used.

Peter Van der Linden makes the case in his book "Expert C Programming":

We analyzed the Sun C compiler sources to see how often the default fall through was used. The Sun ANSI C compiler front end has 244 switch statements, each of which has an average of seven cases. Fall through occurs in just 3% of all these cases.

In other words, the normal switch behavior is wrong 97% of the time. It's not just in a compiler - on the contrary, where fall through was used in this analysis it was often for situations that occur more frequently in a compiler than in other software, for instance, when compiling operators that can have either one or two operands:

switch (operator->num_of_operands) {
    case 2: process_operand( operator->operand_2);
              /* FALLTHRU */

    case 1: process_operand( operator->operand_1);
    break;
}

Case fall through is so widely recognized as a defect that there's even a special comment convention, shown above, that tells lint "this is really one of those 3% of cases where fall through was desired."

I think it was a good idea for C# to require an explicit jump statement at the end of each case block (while still allowing multiple case labels to be stacked - as long as there's only a single block of statements). In C# you can still have one case fall through to another - you just have to make the fall thru explicit by jumping to the next case using a goto.

It's too bad Java didn't take the opportunity to break from the C semantics.

In a lot of ways c is just a clean interface to standard assembly idioms. When writing jump table driven flow control, the programmer has the choice of falling through or jumping out of the "control structure", and a jump out requires an explicit instruction.

So, c does the same thing...

To implement Duff's device, obviously:

dsend(to, from, count)
char *to, *from;
int count;
{
    int n = (count + 7) / 8;
    switch (count % 8) {
    case 0: do { *to = *from++;
    case 7:      *to = *from++;
    case 6:      *to = *from++;
    case 5:      *to = *from++;
    case 4:      *to = *from++;
    case 3:      *to = *from++;
    case 2:      *to = *from++;
    case 1:      *to = *from++;
               } while (--n > 0);
    }
}

If cases were designed to break implicitly then you couldn't have fallthrough.

case 0:
case 1:
case 2:
    // all do the same thing.
    break;
case 3:
case 4:
    // do something different.
    break;
default:
    // something else entirely.

If the switch was designed to break out implicitly after every case you wouldn't have a choice about it. The switch-case structure was designed the way it is to be more flexible.