Why Does 0x appear in Hex value?

0x appears in a hexadecimal literal to distinguish it from other tokens(variable name or functon name).

ABCDE123 could mean either of these things. It cannot be distinguished easily. It contains valid digits to be an hexadecimal number and is also a valid token name.

The creators of C/C++ decided to use a leading 0x.

A leading 0 is not allowed as a valid token name in this laguage. So anything starting with 0 is sure to be a number.

Only having 0 is not enough as it is reserved for signifying octal number. So 0123 is surely an octal number.

0x123 or 0xABDC are surely hexadecimal litreals.