The uses for that single number are as follows:
a) Some class of people like to claim "mine is bigger", which requires a single number. While that is stupid, most people "understand" this type of reasoning.
b) Anything beyond a single number is far to complicated for the average person watching TV.
In reality, things are even more complicated, as speed and compression ratio depend both on the data being compressed, and do that independently to some degree. This means, some data may compress really well and do that fast, while other data may compress exceedingly bad, but also fast, while a third data set may compress well, but slowly and a 4th may compress badly and slow. So in reality, you need to state several numbers (speed, ratio, memory consumption) for benchmark data and in addition describe the benchmark data itself to get an idea of an algorithm's performance. If it is a lossy algorithm, it gets even more murky as then you need typically several quality measures. For video, you may get things like color accuracy, sharpness of lines, accuracy of contrast, behavior for fast moving parts, etc.