Answer:
Think about it like this, if every human had to become a biologist, physicist, mathematician, chemist, historian, artist, poet, athlete, astronomer, lawyer, celebrity, government offical, ETC................. Would anything ever get done, No.
The same applies to multicellular organisms. It is unrealistic/impossible for all cells to perform all functions. But when specialized cells come in to the equation significantly more complex organs and organisms can be formed. So Cell specialization in multicellular organisms allows cells to more efficiently preform tasks.