Can anyone tell me why we use Form and %f in root?
When you need to include a number into a character string.
Do you have a particular example where you need explanations ?
I am not sure I understand your question as it is quite generic. Form allows you to create strings from a template in which you can insert values from other variables. %f denotes that you would like to insert the information of a floating point number. You can as well select how many digits to display:
root  float number(1.23456); root  result1 = Form("The number is %.4f", number); root  result1 (char *) "The number is 1.2346" root  result2 = Form("The number is %.2f", number); root  result2 (char *) "The number is 1.23"
You have the same formatting options as for printf: en.cppreference.com/w/cpp/io/c/fprintf