This movie gets so close to the opening of the original Star Wars that theaters might as well have distributed bellbottoms and cocaine during the closing credits. But wait, what the heck is Leia even doing there? Obviously her ship wasn't contributing to the big space battle we just saw; it was just chilling inside a whole other ship. And was the plan always to give the Death Star plans to Obi-Wan? Wouldn't that be like delivering classified documents to a retirement home?
The nice thing about nonsensical Star Wars moments is that they're all explained somewhere in a book; if a boom mic accidentally popped into frame in the original Cantina scene we'd probably have gotten a whole series of novels about the devious Mikroh'fone creatures of Tatooine. The same thing happened here; in an interview, a Lucasfilm rep explained that the novelization will show how Leia was planning to go fetch Obi-Wan -- not to give him the Death Star plans, just to see if one of the only surviving Jedi wanted to join their team. This was hinted at -- briefly -- in the film itself during a fairly forgettable moment:
"Plot hole, plot hole, plot hole?"
"Plot hole, plot hole."
Apparently the Tantive IV's hyperdrive was leaking and was being repaired aboard the Mon Calamari ship when the surprise attack on the Imperial archives drew them into battle. So that's all. Leia was just chilling out in the Star Wars-verse equivalent of Jiffy Lube when the Bantha poodoo hit the fan.