Why to use ratios to compare two quantities and not difference?

Question

I was listening a lecture on computer performance measurement and the professor was giving an analogy of aircrafts performance measurement. He showed a table which contained different parameters of different aircrafts such as:

Aircrafts:     Passenger Capcity         Speed
Concord              132                1350 mph
DC9                  146                544  mph

then he asked the questions from the students that "How much faster is the Concord compared to DC9?". Then he explained that its more than 2 times. My question is, why did he use Division to compare two values and not Subtraction? I know its a very fundamental question but please excuse my incompetence for that.

Answer 1

I posted the same question on Dr.Maths and got the following response which in my opinion is more precise and elaborated.

Ask yourself which would be more meaningful to you:

  The Concord is 806 mph faster than the DC9.
  The Concord is 2.5 times as fast as the DC9.

If you have no idea how fast the DC9 is, the first statement would be
nearly meaningless -- you can't tell whether it's just a small
improvement (from, say 100,000 mph to 100,806 mph!) or a huge
improvement (from 10 mph to 816 mph). I'm exaggerating to make a
point: interpreting the significance of the number depends on having
at least some knowledge of related numbers.

The ratio, on the other hand, requires no such knowledge.

Also, and perhaps even more important, the ratio will be the same
regardless of the units used. We don't need to know whether the speeds
were measured in mph or kph or inches per second. In effect, the ratio
amounts to using the DC9 itself as a unit of measurement -- the
Concord flies at 2.5 DC9's.

The same is probably true in comparing computer speeds. Who knows,
these days, what is a good speed? But anyone can tell that twice as
fast is a lot better. This is something we can visualize a lot better
than nanoseconds or gigabytes!

Answer 2

Consider a situation - I ate $1000$ apples. My friend ate $1050$ apples.

Two statements- My friend ate $50$ apples more than me from difference, My friend ate $1.05$ times number of apples as me from ratio.

Consider another situation where I ate $100$ apples and my friend $105$

The two statements would be My friend ate $5$ apples more than me and
My friend ate $1.05$ times the number of apples as me

A third situation- I ate $1$ apple, my friend ate $51$

The two statements - My friend ate $50$ apples more than me and
My friend ate $51$ times the number of apples as me

Conclusion - We need both difference and ratio to clearly know the situation. However, we use different things at different scenarios which I hope is clear from the above exmple.