In a stage race, everyone needs to finish inside the time limit to start the next day. So, if you were hired to be the diesel on the flat stages, you have to finish the mountain stages within (say) 20 minutes of the winner. (The time limit is usually a percentage of the winner's time though I believe the race promoters have some say as to how that plays out. And there have been a few famous incidents where the time limit got pushed back because most of the field didn't make it.)

Sometimes making that time limit is hard. I watched film footage of the gruppetto working their tails off to make the cut. The stronger riders were making it a team time trial of desperation. Nobody was having fun. They made it but they went through a private h*** that no fans saw.