In Back to the Future Part 2, Doc practically forces Marty to come back with him to 2015 to correct something regarding his kids. Basically Marty Jr. needs to say "no" to Griff so that he doesn't get jailed. So Marty Senior does his task, gets Griff's gang jailed and his son Marty Jr is free and will not go to jail. Task completed. Go back to 1985 and everything will be just as it was at the end of Part 1. Biff owns a car waxing company; George McFly is a successful author.
Here's what I don't get : What exactly is the point of going to the future and correcting it, and then coming back to the present? You're still going to have to live through it again after you arrive in 1985. It would have made sense if Marty and Doc were planning to live in 2015 permanently (a hard task given that their future selves will be milling about). But just because Marty Jr avoided jail in the timeline featured in the movie doesn't mean he won't once again get the same request by Griff to take part in a robbery through the natural progression of time. (when Marty returns to 1985 and lives out his live the normal way). Marty Jr is once again going to have to say "no", once again get chased on hoverboards, beaten up etc. And who knows if he'll be successful the 2nd time around?