Why should the URL be a constant?

The change I'm imagining would be

```ruby
MACBETH_URL = "http://www.ibiblio.org/xml/examples/shakespeare/macbeth.xml"
HAMLET_URL = "http://www.ibiblio.org/xml/examples/shakespeare/hamlet.xml"
macbeth_analyzer = ShakespeareAnalyzer.new MACBETH_URL
hamlet_analyzer = ShakespeareAnalyzer.new HAMLET_URL
```

I guess I don't see the value.