Why we use url variable

Question

in many projects I saw developer using URL variable by using ? sign, friends what is use of URL variable, please tell us why it is important

Answer 1

Imagine you have code where you use url of some site 100 times. Now you want to change that url. When you use variable you have to change 1 line of code. Otherwise you have to find and change 100 lines of code and pray to not do a single mistake jn process.