Gmail keeps basic CSS, I once received the famous "I recorded your webcam watching naughty stuff!" spam, and the particular attacker "hid" the compromised password (long since changed) in white text between each paragraph, making it essentially invisible.
I suspect to key the email to a particular user and prevent scambaiting? And possibly gather the tiniest bit more information in case someone replies from another email address. (Now they know that the sent email is read and the received is actively used)
Note also: dunno about webmail, but e.g. Thunderbird ignores most formatting when displaying a message marked as spam.