XML Document Using XML Schema?{rep points included!}

[Dillinger4]

This seems very confusing to me so i am hoping that someone might be able to clarify.

Im reading an old book (June 2000) on XML so i don't know if this is still the way to do things. The book shows the following lines. I understand that the first line is a PI instruction to the XML parser. Now the second is the xml schema instance. Third is the xml doc namespace and the forth is the location of the xml schema doc .xsd. Now why is it needed that a namespace be defined for the schema? I understand namespaces as they relate to xml elements but is it the same for the schema,
to avoid collisions? Thanks.

<?xml version="1.0"?>
<addressBook xmlns:xsi="http://www.w3.org/1999/XMLSchema/instance"
             xmlns="http://www.oreilly.com/catalog/javaxml"
             xsi:schemaLocation="http://www.oreilly.com/catalog/javaxml
                                 mySchema.xsd">