Welcome to WebmasterWorld Guest from 126.96.36.199
It's also a good customer service thing to remind people they're being paid from AdSense...
...also wondering why AdSense team would send it so late after the payment was made.
Bill made excellent suggestions in a previous post: The email was for market research or customer engagement. Could be data collection for research because of the tracking codes and single pixel image that phones home to let Google know the email was opened.