Finding an attachment someone sent last week that you didn't have time to look at until now.
Finding a long set of instructions you sent to someone two years ago so you can forward it to someone else.
https://support.google.com/mail/answer/7190
Plus labels and filters allow me to automatically "tag" messages based on sender/domain/wildcards/other variables
I don't have a huge amount of emails but I manage 3 Gmail accounts using a single client (with inbox delegation) and also older Ymail and Hotmail accounts as POP3.
That they use to read your emails, harvest your data and advertise to you.
see https://notmuchmail.org/ your email archive just isn't that big
as for natural language processing, I'm not aware that Gmail does this for user search? maybe it does, but I haven't noticed more than some basic stemming. and given the choice (is not hidden), even most basic users prefer exact/stemmed keyword search over "cleverly" second-guessing returning noise results you didn't ask for.
also there's many non-proprietary search result sorting and filtering techniques that do a great (and predictable!) job, locally, without having to go all deeplearning about it.
remember that a lot of the sophisticated NLP analysis stuff Gmail does, is for the benefit of targeted advertising first and user experience second.