As someone already stated, the BTTF time travel rules function off the basis that if you go to the future, you appear in a future based on the already written history of the place you left. This is a timeline specific to the time-travelers themselves, not to the fact that they traveled in time. Unless they actively change something, any future they travel to would be un-affected by the events in the present, so the future they saw would carry on as though they'd never climbed into a time machine. That's why Marty and Jennifer are in Hilldale with kids instead of non-existent.
As for the travelling back to the past thing, Marty interfering with the car hitting his dad was all it took to ALTER the events of his parents falling for each other and conceiving him. However, there were other ways for this to happen after this incident, which is why Marty didn't fade into nothingness RIGHT when the car hit him instead of George. Because he pushed George out of the way of the vehicle, he immediately changed the future (as evidenced by the fact that his family and home life had changed when he returned), but the point in time that would have him either existing or not existing turned out not to be the car incident itself, but the moment that George kisses Lorraine on the dance floor. She even says in 1985 at dinner that THAT was the moment that did it, not the incident with the car. If George hadn't done it right then and there, she would've given up on him, they wouldn't have gotten married, and Marty never would've been born.
Now, I said all this to finally get to the question at the source of this thread, haha. Old Biff travels back to the past (from his perspective, this is EXACTLY what Marty did in the first film, traveled from the present to the past), and gave the almanac to his past self. This act in itself altered the future, because Biff (in any future incarnation) will remember some weird old guy giving him a sports book that told the future, but he couldn't USE the book to make money until he could legally gamble on his 21st Birthday, so the timeline wasn't dramatically altered yet. However, old biff coming back to 2015 left that timeline on the course it would take without Doc and Marty's interference, which resulted in Biff becoming rich, marrying Lorraine, and getting shot by her. Even after arriving though, Biff still had a chance to go back, because the time machine was still there, but he felt the pain of fading into nothingness already setting in, because Doc and Marty were coming back to the time machine. When they left, he had NO chance whatsoever to fix what he'd done, so he faded into nothingness, doomed by Lorraine shooting him as a result of the change he'd made in 1955. Doc and Marty still hadn't interfered in 1955, they went back to 1985, so their appearance there wouldn't have altered the 2015 information at all. When they jumped back to 1955, their progress and attempts to steal the almanac may have slightly altered events along the way, but we dont see those changes, all we see is the end result, Marty gets the book, burns it, thus leaving NO way for Biff to use it, and it returns the timeline to close similarity to where they left it before old biff stole the time machine.
Now, this doesn't mean that nothing was affected by their actions in 1955 though, for all we know, bystanders to the incidents could've been altered, and their lives could have changed due to Doc and Marty's presence. Again, we don't SEE these things, but that doesn't mean they don't happen. In the novelization of the first film as a matter of fact, Doc asks Marty if he did anything during his arrival and coming to the house that could've altered the future, and Marty says he only bought a cup of coffee and went to see a movie, to which Doc replies that even THOSE small actions can alter time. If somewhere down the line, the owner of the theater had a choice to sell off the theater or keep running it, the ten cents (yes, ten cents to see a movie, can you believe it!?) that Marty spent there that day, might've ended up being the difference between the owner of the theater selling or keeping the place.
So we see other time travel rules coming into play, not just big incidents causing big changes. There's butterfly effect, and fixed point theory all over the place as well.
