Answer:  

Dr. Shimizu Mistuharu, who is studying alloys, kindly answered the question above.
 
 
Pure metal does not have strength. (Except for Titanium. We can make products using pure Titanium. But we still usually use Titanium alloy.) Pure metal also does not have the specific features which we desire. So we add other metal elements to bring out these features.
 
For example, the body of an airplane, which is made of Al-Cu alloy (duralumin), has a streamlined shape to reduce air resistance. The metal has to be soft in order to make such a shape (Al is used as a base because it is soft), however, it would change shape if it remains soft. So we add Cu (and a small amount of Mg and Zn). Then the compound CuAl₂ is created gradually as time passes, and the material becomes rigid. Thus, the material is soft when we make the shape, but it becomes rigid after that. 
 
The mechanism is complicated, so I do not explain it here. 
 
 
Acknowledgement
We would like to thank Dr. Shimizu Mistuharu for his kind answer.