Why do runners use pace to measure speed?

It is definitely weird that runners measure speed with minutes/mile or minutes/km instead of the most common speed measures of mph or km/h. First of all, the higher the number the slower you are!

Instead of saying I run at 6.2mph you should say - if you're a runner - I run at 9:40min/mile. Or, 10km/h becoming 6:00min/km.

Why is it so? It's because pacing is a crucial component of any running training and if you fail at keeping your pace within the ideal limits while you progress one thing is bound to happen: you'll suffer a lot.

Now imagine the challenge of keeping your speed at 6.5mph. How can you get a sense of the difference between 6.2mph and 6.5mph? 0.3mph means nothing to us, let face it, but in terms of min/mile we are talking about 9:13min/mile, that is, you will be doing each mile in 27 seconds less time when compared to 9:40min/mile.

Still hard to get a sense of it, but it gives us the ability to understand that a few seconds per mile (or km) makes a lot of difference.

Let's take an example of a marathon where the runner A runs at an average pace of 9:00min/mile while runner B is only 5 seconds faster per mile 8:55min/mile.

Their final timing will be:
Runner A: 3h55m58s
Runner B: 3:53m47s

Runner B hits the finish line 2 minutes and 11 seconds earlier! That's enough to grab a beer and relax waiting for runner A.

So, if you're a runner, you should abandon mph ou km/h and embrace the beauty of pace. According to wikipedia:
In running, pace is usually defined as a ratio of the number of minutes it takes to cover a mile or kilometer. Pacing is often a critical aspect of endurance events. Some coaches advocate training at a combination of specific paces related to one's fitness in order to stimulate various physiological improvements

After some experience you'll be amazed to realize you can actually tell - approximately - the pace you're running at.

Now, a great article about the importance of pacing by Jeff Galloway:

4 Steps to Your Perfect Pace

By Jeff Galloway

When you run within your limits, every workout can be a pleasure. But start even a few seconds per mile too fast, and misery awaits: excess fatigue, loss of motivation, or even injury. That's why it's so important to know what pace is right for you. Happily, by doing a simple "magic mile" time trial, you can find the best speed for your runs, then set realistic goals and keep running—enjoyably—forever.

Run One Mile Hard
Go to a track and jog an easy lap or two for a warmup. Walk for three to four minutes. Then time yourself running four laps, which is about one mile. Don't run all out; just push a little faster than you usually do. Record your time. By running on a track—which is flat and provides the most accurate measurement of distance—you'll get a solid indication of your top speed. You can use it as a benchmark to determine what pace is appropriate for your current fitness level on daily runs. Do the time trial every two weeks or so; try to beat your previous time, and track your progress.

Slow Down Every Day
On your daily runs, aim to run two to three minutes slower per mile than your magic mile time. So if you do your magic mile in 10 minutes, aim to keep your pace around 12 to 13 minutes per mile on daily runs. At the perfect pace, you should feel comfortable and relaxed—like you can finish a sentence without having to catch your breath. If you're huffing and puffing, ease off. Don't worry about going too slow.

Set Race Goals Signed up for a race? Use the magic mile to set realistic goals for different distances. Add 33 seconds to your mile time to determine a pace for a 5K. Multiply your mile time by 1.15 for a 10K, 1.2 for a half-marathon, and 1.3 to predict your marathon potential.

Get Used To It At a race, you'll get the best results if you try to maintain a steady pace from start to finish. Here's how to practice: Once a week, try to run your goal race pace for a half to three-quarters of a mile. Each quarter mile, check your pace and adjust if you need to. Each week, run a little farther at your goal pace until you're running one-third to one-half of the race distance.

Time Trial
If you can run one mile in 10 minutes, here's your pace for other distances.

Distance: 5K
Pace per mile: 10:33

Distance: 10K
Pace per mile: 11:30

Distance: Half-Marathon
Pace per mile: 12:00

Distance: Marathon
Pace per mile: 13:00