Originally Posted by
cova
Mileage Runs are generally done to earn EQM/EQP (Elite Qualifying Miles and/or Elite Qualifying Points) for the purpose of achieving higher elite levels. M/R'ers generally seek out low fares with maximum distance or greater number of segments.
You can also have a vacation run which combines the milage run (a cheap ticket with maximun EQM miles) with a weekend or longer stay. With a true MR you wouldn't even leave the airport.
Take a look over at the milage run forum to find a wealth of info.