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Re:Gmail Launches Add-ons by DerfNet on Tuesday October 24, 2017 @05:11PM (#15545289)
They explicitly used to scan emails for advertising purposes:

http://www.wired.co.uk/article/google-reading-personal-email...

Re:Gmail Launches Add-ons by icebraining on Tuesday October 24, 2017 @06:12PM (#15545672)
Hence any other reason. Advertising was the reason already in discussion.