Srigurubhyo namaha,
Hi,
May be this information could be of some use, Shraddhams come form the word shraddha, So it will have no meaning if we just say shraddha without indicating anything. When you have pitr abbdhika Shraddha[ even amavasya, Tarpana days et al],
You are in a vow[ to your pitru] that day, so you need to complete the Shraddha first, because it's the instruction of bhagavaan[ smriti,shastras] to complete pitr vows first and then attend deva karyams, as devata are fulfilled one and pitr are not, so that's why even bramha Yajna is done after shraddha.
The logic behind here is you are directly descended from them, so if any Bhagavath vishaya etc, are done is because of this birth. So, you are worshipping paramatma [only] through them. Just like worshipping vishvaksena or Ganapathi[vighneshwara] before any puja/event.