Every workout you do as part of a training program should have a purpose, intensity, time/distance, and structure (PITS) associated. Even if the purpose of a workout is “to clear my head” or “to shake out my legs”, those are valid reasons and have their place. The PITS will vary according to where and how the workout fits into the larger period or cycle in a training program. I should mention an exception here, sometimes you just want to run for no reason, with no predetermined intensity, time, distance, or structure. That’s the beauty of flexible training plans, there’s even a place for those care free runs.
There are many structured workouts that can be applied. The positive split is one such structure where the first half of the run is faster than the second half. In marathon training, one approach is to break up the demands of the goal event into smaller pieces, that become progressively more compressed over time. You can simulate small pieces of the goal even by training at the intensity you want to run your goal race in training, using clever techniques to get used to how it feels to run at a prescribed intensity (pace) toward the end of a marathon. The positive split is one example of simulating the demands of the race, by using the first half to induce a significant fatigue, than requiring the second half to be run at goal race pace. As a race strategy, using a positive split can be beneficial as well, but the decision to use this in a race must be carefully weighed, it does have potential to backfire, leaving the athlete with little to no gas left in the tank for the final push.
There is a time and place for both negative splits and positive splits and even splits all throughout a training program. The beauty of training plans is that they are as unique as the individuals who use them.