Because its faster. Its generally accepted as a best-practice throughout the JavaScript world to do array.length-caching.

See:
- Google Closure Library's [`array.js forEach function`](https://code.google.com/p/closure-library/source/browse/closure/goog/array/array.js#193)
- http://stackoverflow.com/questions/3010840/loop-through-array-in-javascript
- http://www.erichynds.com/blog/javascript-length-property-is-a-stored-value