getUniqueValue() doesn't take any arguments, and it is a quick and easy way to retrieve the sys_id of a given record.

getValue() on the other hand, returns the value of a specific field or column. This can be the sys_id column, or any other column that might contain a value:

var gr = new GlideRecord('incident'); 
gr.get('46f09e75a9fe198100f4ffd8d366d17b'); 
var sysIDVal = gr.getValue('sys_id'); 
var incNumber = gr.getValue('number');