MW - it's an oft asked question! Escapados have different reasons to go off. Sometimes it's to get tv coverage for the sponsor (especially local media companies in smaller races), sometimes it's part of a plan to break up the peloton by one team or more, sometimes it's just down to the riders who are real racers, and want to see what the others are made of eg David Millar (Garmin), Jens Voigt (Trek Factory Racing), Tommy Voeckler (Eurocar), Tony Martin (Omega Pharma Quick Step). So sometimes they know they will be swallowed up but that's okay. It's a team sport. Today's course, like yesterday's was made for a break, but today the big sprint teams didn't let anyone get too far away as the end was a sprinters inviation.
If the breakaway doesn't contain anyone threatening the over all lead, the General Classification for the yellow jersey, then the big teams in the group will let them have their fun eg Tony Martin a couple of days ago, or Gallopin yesterday.
The peloton always has the advantage by riding in a group and so expending less energy than the escapers. IF the breakaway contains experiences riders working together in sharing the load i.e. taking turns at the front, and they don't break up, then the group might get nervous and up the pace to catch them.
It's often though the poker game in the peloton - who is going to sacrifice riders to do the chasing? So whilst they are looking at each other, the escapados might just keep plugging away to victory!