Because you want to train at your goal 5k pace (4 min/km if you are aiming for 20 minute flat 5km).
For example, you can run 2.4km at 9.36 minute (4 min/km). You can 100% complete 4 x 800m (4min/km) with 2 minute rest. Then next thing is to do 5 x 800m at the same pace and rest, so it will be harder every week. Continue until you hit 6 x 1km (4min/km) with 2 minute rest. I tested 6 x 1km with 2 minute rest a few times, and it reflects your 5k race pace. If you are aerobically weak and fast speed, then try 5 x 1.2km with 2 minute 30 seconds rest.
8 x 1km @ 60s rest is only possible if you run at 10-15k race pace. Unless your goal is to focus on 10-15k pace as well as 5k, then it's just better to do shorter intervals and get good at it for that 3 months.
Of course, you die die want to do 8 x 1km @ 60s rest, you can improve by running a faster average pace or cutting down rest or both across the week. The major downside is that you wouldn't be running at your 5k pace.
I find shorter intervals useful because you need a lot of speed for 5k. You can get the endurance training from long run. Add in 20-30 minute tempo at the end of your long run (negative split), you are basically training speed endurance.
Speed (short intervals with longer rest) + Endurance (Long run) + Speed Endurance (20-30 minute tempo - can be done separately or in long run) = PB
You don't need to do 8 x 1km, you can do 6 x 800m (2 minute rest) >> Do shorter intervals, which allows you to run a better tempo at the end of the week since your legs are not as tired.
