One of the purposes of breaking up a total volume of running into reps with rest periods is to get as much stimulus (and practice) as possible at a particular pace before getting into trouble.
Doing 20 laps in 68 seconds each with 45 seconds intervals (traditionally, "interval" refers to the break between bouts, not the bouts themselves) is generally easier than doing 10 laps in 68 seconds each with no rest periods between the laps. The heart, lungs and skeletal muscles get a more robust stimulus from the larger volume of work than it does from the shorter continuous run, and with less chance of lowering muscle pH or blood pH. The higher number of reps is also better for promoting a sense of rhythm - hence, the reference above to "practice."
Doing 10 reps at a faster pace (presumably with slightly longer rest periods) increases the chance of lowered pH, which can be a useful training tool in moderation but is easy to overdo. Speeds near 3k race pace tend to better promote mitochondrial biogenesis than speeds at 5k race pace. The faster pace also obviously invokes different muscle fibers and provides some variety in mechanics and impact stress distribution.
20 laps at 5k pace =
* More profound stimulus to the heart, lungs and skeletal muscles
* Normally less sudden reduction (and less end reduction) in pH
* Easier to find the correct tempo and control it
* Familiarity with running 5k pace
10 laps at 3k pace or faster =
* More stimulus for mitochondrial proliferation
* Variety in 1) fiber recruitment, 2) range of motion in muscles/connective tissue, 3) torque and associated impact distribution stress
* Familiarity with running the faster pace
There are plenty of other benefits and drawbacks to using a variety of speeds for interval training (some of which obviously overlap), but above are just a few of the differences between those particular paces.