Is a default case necessary in a switch statement in C programming

No, but it is not a bad idea to put default statements in switch statements for error- or logic-checking
purposes. For instance, the following switch statement is perfectly normal:

switch (char_code)
{
             case ‘Y’:
             case ‘y’: printf(“You answered YES!\n”);
                                break;
             case ‘N’:
             case ‘n’: printf(“You answered NO!\n”);
                                break;
}

Consider, however, what would happen if an unknown character code were passed to this switch statement.
The program would not print anything. It would be a good idea, therefore, to insert a default case where this
condition would be taken care of:
...
            default: printf(“Unknown response: %d\n”, char_code);
                                break;

Additionally, default cases come in handy for logic checking. For instance, if your switch statement handled
a fixed number of conditions and you considered any value outside those conditions to be a logic error, you
could insert a default case which would flag that condition. Consider the following example:

void move_cursor(int direction)
{
switch (direction)
{
              case UP: cursor_up();
                         break;
              case DOWN: cursor_down();
                         break;
              case LEFT: cursor_left();
                         break;
              case RIGHT: cursor_right();
                         break;
              default: printf(“Logic error on line number %ld!!!\n”,
                                                __LINE__);
                         break;
}
}

Cross Reference:

When is a switch statement better than multiple if statements

Can the last case of a switch statement skip including the break