If you read a lot of emails every day, you probably don’t want to open each one to find what it’s about. On iOS you can actually expand the preview so you don’t have to open each one.
Head to the Settings App, and then press “Mail, Contacts, Calendars” on the left-hand side. Under the “mail” category, hit “preview”. There, you’ll be able to change the preview from none, to five lines, depending on your preference.