You need to determine what the value attribute is for a field name.
$value = Form::getValueAttribute($fieldname);
This will return the value that should be used for the field or
null if there's no value.
You can also specify a value that takes precedence over the model value if your form is bound to a model.
$value = Form::getValueAttribute($fieldname, $value);
If the flash session data contains a value for this field it still takes precedence, but passing the value as the second argument will take precedence over the model data.
This is often used in form macros.
There's a precedence to assigning values to fields. This precedence is discussed in Creating a New Model Based Form.
Form::getValueAttribute() allows you to use the correct value, following the precedence, within your form macro.
See the Creating Form Macros recipe information about form macros.