On your Master Promissory Note (MPN), there is an optional line for you to enter your email address. Sometimes, students ask why they should enter it if it's optional. This is an option that is highly recommended you fill in.
Loan servicers are switching over to electronic services and messaging. You can pay your student loan bill online and manage your student loan account online. Email is just the next step in that. Loan servicers are trying to cut down on costs, and mailing out statements costs money in postage and paper, not to mention the ink to print them. Although they will mail you statements, they actually prefer to email statements which keeps their costs down.
Also, it's an attempt to meet the newest generation of student loan borrowers on their own terms. As technology continues to grow and the face to face conversations (and even voice to voice conversations) are going down, the student loan servicers are trying to converse with the borrowers in an impersonal manner such as email. So for those that have developed a phone phobia, the servicers are willing to accommodate for that, leaving no excuse not to get in touch with them. The servicers want to hear from the borrowers, and email is just another way for that to happen.