Visual reminders play a bigger role than verbal ones when we’re trying to remember something.
It's probably why my mom texts me something almost daily instead of calling (e.g. "Don't forget your second cousin's birthday"). Or why milk cartons became PSAs. Or why T-1000 terminators carried a Polaroid picture around even though they were robots that could travel back in time.
Emails, as you guessed, are a great visual cue.
They serve as reminders of all kinds. Reminders to complete a purchase. Reminders of upcoming events. And, for many in the US who received plentiful amounts of 4th of July sale emails today, they are great for limited-time discount reminders too.
Why do we get so many reminders in our inbox? Because they work; they connect brand, loss aversion, and urgency in one single message. And RGE has over a hundred examples to inspire you for your next campaign.