Why would you make it constant? To protect it against other programmers?

Answer 1

Sometimes you need a constant like PI to use it several times in different statements. Using constant prevents it from being changed by mistake and also improve memory usage efficiency.

Answer 2

beacuse it should be constant no other thing

Answer 3

No one, including a god and yourself, should manipulate numbers like the average constant of earth gravity. Huiiiii! ;)

Answer 4

Because thou shall not change it in your program