This option sets the selected attribute to an empty value. The existing attribute value is replaced by a NULL string.
Normally you won't need this option very often. Many directories generally prohibit the LDAP clients to set empty values to attributes (for example Microsoft Active Directory), other directories doesn't allow you to set empty values for the most attributes (for example OpenLDAP), so that you may receive the following error if you try to use this option:
Maybe you should remove the complete attribute instead in this cases. However, there are situations where attribute values are necessary: Imagine an LDAP object class which have a certain attribute as a must attribute, so you can only create a new object of this class if you set the attribute. Now: If you don't want this attribute to have a value, but it's a must attribute, then you have to set an empty value!
An example for such a situation is the objectclass 'group' in OpenLDAP environments. If you want the group to have NO members, then it's impossible to remove the 'members' attribute, you have to set an empty value instead, because the members attribute is a must attribute for this object class.
If you want to set an empty value, you can also try to select the according row in the attribute list, edit this line and set an empty sting for it. But this may result in an error if the attribute is a multi value which as already more than one array value. So the Set Empty Value option can replace the entire attribute with a NULL string in this case.
By the way: If you see the Set Empty Value option disabled and gray, then your current application focus is not in the attribute list or your LEX is maybe in Read Only mode.