It's afternoon a motley collection of droids are lined up on the Moisture Farm, in the shade of a Jawa Sandcrawler.
Owen decides to buy Skippy the Jedi Droid (R5-D4) rejects R2, and then chooses Threepio. But when R5 blows his motivator Threepio suggests taking R2 as a replacement.
Fast forward a few moments to when Luke is leading them into the garage.
THREEPIO- "Why I should stick my neck out for you is quite beyond my capacity!"

Alright, here's the kicker. What if Threepio didn't stick his neck out for R2?
What if he'd remained silent and let the Jawa's take R2 away on the Crawler. What would have happened?
It's concievable that the Empire would have found the Crawler, killed the Jawa's and taken R2, therefore retrieving the plans for the Death Star and stopping any possibility of an attack on the station.
They wouldn't have had any need to go back to the Moisture Farm and kill Beru and Owen.
Ben would never have learned about the mission to Alderaan, and therefore he couldn't ask Luke to help him with the operation.
And without the violent deaths of his adopted family, Luke, in his indecision, would have a reason to stay on Tatooine - his aunt and uncle.
If it had played out that way, Ben would have needed to tell Luke a lot more about his heritage in order to persuade him to leave Tatooine, because at some point, Ben and Yoda would need to bring Luke, the heir to the heritage of the Jedi, into the field of play in order to defeat Vader and the Emperor.
So it appears that the fate of the Jedi, the fate of the galaxy and the destruction of the Death Star were all closely linked together.
But if R2 had never managed to deliver the message to Kenobi, none of it would have likely happened.