A value accessor is a component that associates a value reader with a value writer.
An example of a value accessor is a value accessor used to read or write the a field on a Sitecore item that represents a product.
A value accessor is like a .NET property. A value reader is like a property getter. A value writer is like a property setter.
Custom value accessor templates make it easier to configure a tenant. Consider the requirement to support reading and writing properties on CRM entities.
The act of reading and writing different properties only differs by the name of the property being accessed. A custom value accessor template lets users specify the property name once. The value accessor itself is able to create the appropriate value reader and value writer for the specific property.
The need for the value reader and value writer still exists, but the need to configure each separately does not.