Your letter helps more than your grandchildren and yourself. It also helps you children. As they read your life stories, they will know some of them, but probably not of your early stories. Those stories will be new to them. They will learn things about you that they never knew. They will get to see different sides of your life. 

Writing is deeper than talking. Written words are generally deeper than spoken words. As you children read a story you have written they will see a deeper meaning in your experience.

It should also provide them an example of how to write a letter to their own grandchildren. It will want to make them continue the unbroken chain of letters so each generation will be able to look back at their families past with no missing pieces. They can see patterns and relationships.

They can see how bad times make strong people. How strong people make good times. How good times lead to weak people. How weak people lead to hard times. 

So many times life is repeats itself and when you can see those cycles you will be much better off.