random.randint() call with -1?

Can someone explain why the range in the random.randint(0, len(messages) - 1) has a minus 1 at the end? The list has 9 values:

import random

messages = ['It is certain',
            'It is decidedly so',
            'Yes definitely',
            'Reply hazy try again',
            'Ask again later',
            'Concentrate and ask again',
            'My reply is no',
            'Outlook not so good',
            'Very doubtful']

select_number = random.randint(0, len(messages) - 1)
list_select  = messages[select_number]
print(list_select)