Howdy Hey There!
Legally, I cannot answer exactly why tax returns are required, but I can provide suggestions as to why you may WANT to.
If you happened to have had taxes withheld from your pay, you are required must file a tax return in order to receive a tax refund. If you qualify for this, you must file a return to receive the refundable Earned Income Tax Credit. :)
Hope I at least helped a little, Have a great day.
-Ash