Why do I have to add var to define I Variable _ javascript in jsfor loop?

I know that some people (such as me) are not used to adding var when writing js for loops. This is of course permitted by syntax. For example.

The Code is as follows:

For (I = 0; I <10; I ++) {// It is not written as: var I = 0
Alert (I );
}

However, this is really not a good habit. Next I will explain why var must be added to the Js for loop, otherwise it will bring you annoying and difficult to query bugs from time to time.
For example, we want to implement the following functions: Output
10
20
30
40
50
60
70
80
90
100
Using the following code, WriteNumber starts from 1 to 10 cycles, and the TenTimes method is called each cycle to return 10 times the index value.

The Code is as follows: