INTRODUCTION TO STRINGS why use len to get last characters in strings?

In. the class for Python strings in the Data Science course, I am checking this:
INTRODUCTION TO STRINGS

More and More String Slicing (How Long is that String?)

https://www.codecademy.com/paths/data-science/tracks/dsf-python-fundamentals-for-data-science-part-ii/modules/dsf-python-strings/lessons/introduction-to-strings/exercises/more-string-slicing

Question is, why do I need to use len to slice the characters using index number? I was able to run the exercise with no use of len since I considered that was not really needed. I understand it if I need to get the value for how many characters or use that value, but for that exercise where I just need to know character in specific indexes, is that really needed?

This is how I completed the exercise:

first_name = "Reiko" last_name = "Matsuki" def password_generator(first_name, last_name): pwd = first_name[-3:]+last_name[-3:] return str(pwd) temp_password = password_generator(first_name, last_name) print(temp_password)

ha, that was fast, next exercise is what I did: https://www.codecademy.com/paths/data-science/tracks/dsf-python-fundamentals-for-data-science-part-ii/modules/dsf-python-strings/lessons/introduction-to-strings/exercises/negative-indices