...especially if you look at your T-Pace (threshold pace) and realize that if you stick to the 5a side you might be pushing harder than you can theoretically hold for very long! Can we say above lactate threshold?!?!
These workouts may start to appear in the Build phases (once you've gone through your Base phase(s) and have a solid aerobic engine in the works) and become more frequent near the end of Build 2 and into the Peak phase.
SO...why do it?
Swimming at threshold or slightly higher (faster) increases your anaerobic endurance, something sprint and olympic distance triathletes need the most due to the shorter durations of their races. BUT if you train for longer distances (70.3 or Iron distance) these workouts can also be extremely important for you. By training your anaerobic system you have the ability to increase your aerobic capacity (stronger to go longer!) and economy. Double win!
So, each of these workouts usually starts with a Race Start set (with short rest intervals). These are usually anaerobic and at a pace that you can not sustain for long (think 5b). Imagine the race start area and all of the athletes sprinting out to get a good position in the water. Here you get to practice how that feels so you know those first few hundred meters of the race are going to be super intense, but you are prepared for that feeling.
The next set is longer and the meat of the workout (200s-300s at 4-5a pace). The rest intervals are a bit longer here to give you adequate time to recover so the next interval can be done at the same effort. Here your heart rate is going to spike and you learn how to continue swimming at that faster pace for longer while controlling your breathing.
In swimming thise workouts can be the most beneficial (i.e. see the most results) on race day. Because the swim always starts out at a faster pace that you can most likely sustain for the duration, it is important to learn how to go out fast and confidently (anaerobically)! This fast pace usually lasts for several minutes until the crowds thin and then you can settle into a pace that is optimal for you for the duration.
So, the next time you see this workout on your schedule don't dread it! Embrace it knowing that this will get you prepared for race day.