Why does roasting add a flavour to the peanuts?
That great taste comes from the compounds produced by the Maillard reaction.  It not one reaction but many that occur when the building blocks of proteins and sugars react as food is heated.  Many new compounds are produced giving the cooked food a richer range of flavors.  The pyrazines produced by the Maillard reaction give roasted peanuts their characteristic aroma, in particular cyclohexapyrazine.

To produce pyrazines, you need to heat your peanuts above 70C, with 110C to 170C (230F - 340F) suggested by most cookbooks.  For unshelled peanuts at 110C it can take up to ten minutes for the amount of pyrazines produced to reach its peak.  If the peanuts are not dry, you may need to add up to 20 minutes of oven time (longer on a tawa) at a lower temperature to dry them.   Color will be your guide.  The flavor of the pyrazines will be masked by oxidizing oils in the peanuts, so keeping them in an airtight container helps preserve that nutty-cracker-peanut flavor.