Added a note about why you would need this

README.md

@@ -9,6 +9,10 @@ A tool for creating credentials for accessing S3 buckets
 
 For project background, see [s3-credentials: a tool for creating credentials for S3 buckets](https://simonwillison.net/2021/Nov/3/s3-credentials/) on my blog.
 
+Why would you need this? If you want to read and write to an S3 bucket from an automated script somewhere, you'll need an access key and secret key to authenticate your calls. This tool helps you create those with the most restrictive permissions possible.
+
+If your code is running in EC2 or Lambda you can likely solve this [using roles instead](https://aws.amazon.com/premiumsupport/knowledge-center/lambda-execution-role-s3-bucket/). This tool is mainly useful for when you are interacting with S3 from outside the boundaries of AWS itself.
+
 ## Installation
 
 Install this tool using `pip`: