Email has evolved from a messaging service to a multi-purpose tool. Its most common use today is marketing using mass email services.
The original purpose of email was to allow for the instant sending and receiving of messages. Pretty similar to SMS, except it was (and still remains) free. However, this was by no means its only purpose. Email was also used to transfer files and receive newsletters and announcements. As for the latter, an evolved form is still going strong. Added to that are electronic bills and documents, which keep the planet greener, not to dive deeper.
Another original purpose of email that remains evergreen is identity verification. A great many web services require email notification and also use emails to notify the user about novelties and updates to their accounts.
Finally, business communication still relies heavily on the email technology for message service, albeit there are alternative web services serving the same purpose.