what is the use of default in switch-case?

It is similar to tht 'else' in an 'if' statement. Something to do if no specific alternative matches.

Don't you have a good C book to read? If not: Do get one!

to catch coding mistakes
with few exceptions, if default: get entered you have made a mistake

Erik

Entering the default clause in a state machine (where the switch variable is normally an enumerator) should preferably be coded to represent an error.

But a switch statement is used for way more than just state machines.

When decoding charactesr in a parser, you may have:

switch (ch) {
    case '(': do_lparen(); break;
    case ')': do_rparen(); break;
    case '+': do_add(); break;
    ...
    default:
        if (ch >= 'a' && ch <= 'z'
        ||  ch >= 'A' && ch <= 'Z') {
            do_alpha(ch);
        } else if (ch >= '0' && ch <= '9') {
            do_digit(ch);
        } else {
            invalid_character(ch);
        }
}