Form 16 is a type of Certificate given by employer to his employees that he has deducted their TDS and also it gives complete information about Salary and deductions allowed.
Income tax return is to be filed by the person who receives/earns income. A person may also have income other than salary on which tax has not yet been paid or short paid so you have to file your return.
Income tax returns are used as proof of income when you apply for Loan. form 16 is not acceptable there.