1: Normally tax return is filed to claim the excess tax which you may have paid in a given year. Generally, as in your case also, the employer would deduct the tax amount from the employee's salary which could turn out to be more or less than what actually should have been paid by the employee as tax in a given year. If you have paid less tax, you may need to pay more tax on top of what your employer may have deducted out of your salary. I'll give you an example to make things clear for you.
Suppose your monthly income is 20,000 (and your employer deducts 2000 out of it and pays you 18,000, then at the end of the year, you'll be assessable on:
20,000 x 12=240,000 (assuming thats your only source of income)
rather than 18,000 x 12=216,000
so based on the resident income thresholds, you'll be taxed accordingly, e.g.
0 to 6000 p.a exempt from tax
6000 to 30,000 p.a 15% tax
……..and so on so forth
Now based on this, you calculate your annual tax, if your employer has deducted more than what you are liable for, file a tax return
2: have no idea about that.
3: if you dont file a tax return, you wont get your money back in case you've paid more, simple.
But in case you owe something to the taxation authority, you could face legal action if you fail to pay the balance.
P.S. my knowledge is based on the Australian taxation law, but still you can take a general idea from that.
Cheers!