Email Archival is the process of withholding all legitimate emails sent to, from and within your organization for a variety of purposes, but most often as a requirement for regulations stemming from laws passed at the federal level. These emails are used as the primary content for Discovery (the legal term for data recovery, extraction and presentation) when an organization inevitably receives an inquiry or lawsuit, both from internal employees or external organizations.
When not being used for regulatory purposes, archives are also often used for internal investigation, email content monitoring, tracing back on contracts that may be unfulfilled and even to protect an organization from a lawsuit filed by their own employees.
The key difference between a backup and an archive is the requirement from many of these regulations regarding non-spoliation of the data, i.e. an inability to modify or otherwise delete the data prior to its regular purge schedule, as defined by the law. Backups are designed for eventual overwrite and often contain data that has changed over time, most often by deletion from the user's mailbox. Such occurrences never occur on an archive, making it a true compendium of the organization's email history and thus suitable for e-Discovery purposes.
Remember that an archive is different than a backup: archivers store all data continuously, immutably and for as long as your document retention schedule requires; backups come and go frequently and only contain what users had in their mailbox at that point in time, which is usually not everything.