Q:

In India, does a company compulsorily have to file income tax on my behalf? 

I work in an IT company in India.  I get my monthly salary after monthly tax deduction as per TDS law.  The tax filing deadline as per Indian government is 31st July, but my company has a deadline for submitting investment proofs much before it say 15th Dec. Why have they imposed this artificial deadline?  Say, I didn't give them any investment proofs then they would withhold more tax?.  If say I want to file my own tax before 31st July deadline with proofs of investment claiming tax refund, can I do it?  In this case, I would need to ask form 16 from company, right? 
So as per Indian tax government laws, if I am a salaried employee, can I file my own taxes and opt out of filing by company or I have to get it compulsorily filed it by company (which is source of income) and declare all my investments to them? 
Or is it just that company has to withhold some tax from my income as they give me salary and not necessarily have to file my tax.  In such case, I can take form 16 from company and file my own tax?  In this case government will have many correspondences from various sources about the same pan number.  The company telling them how much tax they deducted at source, the individual telling them how much tax its employer paid (through form 16) and telling them his/her investment and then claiming for either balance return or paying more taxes (if he has more income than his salaried income from employer say from property income etc).  Doesn't the government prefer to have a single correspondence with a single entity about tax on a particular pan number?  What if I earn salary from more than one employer say one full-time job and other is part-time job or say one is a regular job another is acting/modeling assignment?  In such cases government will have many correspondences from many places on a single pan no.  Isn't this unwieldy from government point of view? 
What is the difference between filing for income tax payment and filing for income tax return? 
Thanks,

A:

The company is liable only to deduct tax at source.  The company never files Returns.  It can facilitate by arranging a 3rd party. 
The Individual is responsible for filing the Returns. 
Incase the tax deducted is more; this can be claimed as a Refund at the time of filing returns.   
Deducting the tax at source is a way by government to ensure that tax leakage is reduced as an Organisation is made liable to deduct and deposit it with Govt.  This ensures that Govt gets the tax on time.   
As for majority of the cases; the only source of income is Salary, the company includes the deductions allowed by Govt and calculates the tax accordingly. 
Some companies go the extra mile and also request the employees to furnish any additional income and calculate tax accordingly.  They are not bound to do this. 
Incase an individual has multiple sources of income [2nd job, Rental Income, Profits from Shares/Capital Gains/Interest Income], its the liability of the individual to calculate his total income and pay tax accordingly.  The tax has to be paid in advance. 
At the time of paying the tax, there is no declaration required as to what the income was or the source of income.  The Returns is the place when final calculations are to be made showing the total income for the year and the total tax paid for the year. 
It the mandatory for individuals receiving taxable income to file returns.  Once an individual has filed returns, he must file it for subsequent years even if in the next year the income is Zero.  This is typically called as NIL Returns. 