Retcon job, and not a very good one
Opening crawl for New Hope (1977) says that Rebels achieved their first victory against Empire striking from their secret base, and that during battle Rebels spies managed to steal plans for Death Stars. Leia is now custodian of the plans (1:25 in clip) and she races home with them. Home, of course is not Tatooine but Alderaan. Therefore, she didn't come for Obi-Wan specifically. But, it looks like her ship could not make to Alderaan (damage probably ? ), so she puts plans inside R2-D2 and instructs him to find Kenobi because she knows that he lives on Tatooine.
Message by Leia is a bit confusing, she says that her mission is to bring Obi-Wan to Alderaan to Bail Organa. But we could strike that down as diplomatic white lie - Kenobi certainly didn't serve Organa in Clone Wars. She is most likely trying to establish emotional link .
Final piece of the puzzle is Vader himself, mentioning transmissions .
“Several Rebel transmissions were beamed aboard this ship from Rebel spies”
This implies that Tantive IV received plans remotely, and was not part of "first Rebel victory". Full story, before retcon in Rouge One, would be something like : After first Rebel victory, Leia's ship posing as diplomatic vessel, receives transmissions from Rebel Spies containing Death Star plans . Unfortunately, Vader discovers transmissions, and starts pursuing Tantive IV which gets damaged in the process and cannot continue to Alderaan. Leia remembers that Obi-Wan lives on nearby Tatooine, and sends plans to him.