This is an optional ‘empty’ graphic to show that the InputField text field is empty. Note that this ‘empty' graphic still displays even when the InputField is selected (that is; when there is focus on it).
A placeholder graphic can be used to show subtle hints or make it more obvious that the control is an InputField.
If a Text component is used as the placeholder, it's recommended to make the placeholder text look different from the real text of the InputField so they are not easily confused. For example, the placeholder text might be a more subtle color or have lower alpha value.