This workout is specifically used in the early base phase of a training schedule to elicit VO2max or aerobic capacity gains.
The target for these longer intervals are 110% to ensure that riders are not bumping into their anaerobic stores to complete the workouts. Simply put the longer we can stay aerobic for these harder efforts the more you will have left in the tank at the end of a race or ride for a hard finish.
Each workout will have progressive time spent to at this limit to help increase rider FTP as well.