The idea behind multivalued fields is to make it easy to support those instances where you want to select and store more than one choice, without having to create a more advanced database design. Multivalued fields are also important for integration with Windows SharePoint Services
because SharePoint lists also support multivalued fields.
You might wonder why Office Access 2007 allows you to store more than one value in a field, when most relational database management systems forbid this. The answer is that
the database engine in Office Access 2007 doesn't actually store the values in a single field. Even though what you see and work with appears to be a single field, the values are actually stored independently and managed in hidden, system tables. The Access database engine handles this for you, automatically separating the data and bringing it back together again to surface the values in one field.
Technically speaking, an Access multivalued field models a many-to-many relationship. For instance, consider an events table where you assign responsibility for each event to one or more of your employees. Suppose you create a multivalued field named "AssignedTo" for assigning events to employees. The relationship between events and employees is many-to-many. That is, you can have many employees assigned to any one event, and you can also have any one employee assigned to many events.