Well, sure, but the question is how you do "123 * 456". In binary you have the multiplication

   111001000
  x  1111011

and the traditional grade-school algorithm looks something like this

         111001000
        1110010000
       00000000000
      111001000000
     1110010000000
    11100100000000
   111001000000000
   ---------------
  1101101100011000
  122222221

so the answer is 1101101100011000, or 56088 in decimal. This method takes O(n^2) bitwise multiplications, where n is the number of bits in the larger of the two integers. Clearly if you're multiplying very large integers (say you're doing cryptography - the kind of computation that computers do millions of times every day) then there's clearly an interest in having faster algorithms for multiplication.