It then says if we haven't reached that number to go ahead and do whatever it is we're up to. If I had wrote it using the 10 as the check value, we would have very little indication as to what I was checking for. If you use a constant in only one place, it is nice to know what the constant is refering to. If you use a constant in more than one place, it is nice to be able to change it in only one place and have it replaced everywhere else. Acronym's should never be used to name a variable.
Variables have what is refered to as scope.
The scope of a variable is directly related to the amount of other code that has access to the variable. Generally the smaller the scope of your variables the better off you are. That is why global variables are frowned upon. If a variable has a large scope, many functions in your program can directly access them and in doing so change them. Variables should only be changed by functions that are intimate with the variable.