Why do we have to use break in switch?

Question

Who decided (and based on what concepts) that switch construction (in many languages) has to use break in each statement?

Why do we have to write something like this:

switch(a)
{
    case 1:
        result = 'one';
        break;
    case 2:
        result = 'two';
        break;
    default:
        result = 'not determined';
        break;
}

(noticed this in PHP and JS; there are probably many other languages using this)

If switch is an alternative of if, why we can't use the same construction as for if? I.e.:

switch(a)
{
    case 1:
    {
        result = 'one';
    }
    case 2:
    {
        result = 'two';
    }
    default:
    {
        result = 'not determined';
    }
}

It is said that break prevents the execution of the block following the current one. But, does someone really run into the situation, where there was any need for execution of the current block and following ones? I didn't. For me, break is always there. In every block. In every code.