A combo box cells value has to be in the items collection, otherwise a Data Error event will be fired, so that's the reason why you need to perform this step.
public ref class Control : System:: Component Model:: Component, IDisposable, System:: Component Model:: ISynchronize Invoke, System:: Windows:: Forms:: IBindable Component, System:: Windows:: Forms:: IDrop Target, System:: Windows:: Forms:: IWin32Window[System. Control Accessible Object classes, providing as much accessibility information as needed. For more information about the Windows common controls, see General Control Reference.
Sometimes you will want to provide additional information to accessibility client applications.
Windows Forms has accessibility support built in, and provides information about your application that enables it to work with accessibility client applications such as screen enlarger and reviewer utilities, voice input utilities, on-screen keyboards, alternative input devices, and keyboard enhancement utilities.
If the control does not have a Parent, and the property is not set, the control attempts to determine the value of the ambient property through the Site property.
It provides a window handle ( is a control property that, if not set, is retrieved from the parent control.
But you could have a request from the client who wants to have enabled typing in a Combobox cell.