Why do we start with var html=" "; ?

The reason why is because the variable is going to be a string variable that will be used in a loop. Since the variable is going to be a string, we can define it with an empty string value to begin with and add to the value within the loop each time the loop runs. If you were to define this variable inside of the loop the previous loop run where the value will be updated/added will be removed, this is because each time the loop runs, the value of html is being reassigned rather than updated. So when it gets to the end of the condition (which is i being 10), the actual value of html is 10.

You can test this yourself by placing the variable expression inside of the loop and printing to the page, you will see only one div being outputted with the value of 10.