setAttribute() is a DOM 1 function. setAttributeNS() is a DOM 2 function that resolves the problem of conflicting tag or attribute names by specifying the xmlns namespace that should apply to the tag/attribute in the first argument.
If an attribute does not have a defined namespace prefix, the first argument must be null. You could use setAttribute() but for consistency it is advisable to stick to setAttributeNS(). See:
https://developer.mozilla.org/en/docs/Web/SVG/Namespaces_Crash_Course#Scripting_in_namespaced_XML
“However, note carefully: the Namespaces in XML 1.1 recommendation
states that the namespace name for attributes without a prefix does
not have a value. In other words, although the attributes belong to
the namespace of the tag, you do not use the tag’s namespace name.
Instead, you must use null as the namespace name for unqualified
(prefixless) attributes.”