You need to reorder your animations mainly because you might have more than one animated object on a slide, and you discover that some re-sequencing of animations as they happen in relation to each other may provide a better result. For something of this sort, PowerPoint provides a Reorder option that lets you play with the sequence of the animation.
Learn to reorder animations on your slides in PowerPoint 2007, 2003, and 2002 for Windows.

