By default, animation effects are numbered in the order in which they are applied to slide objects. You might need to re-order your animations mainly because you may have more than one animated object on a slide, and re-sequencing of animations as they happen in relation to each other may provide a better result. Or you may just want some animations to happen before the others. Also, there are logical reasons to re-order animations since typically entrance and exit animations need to be the first and last animations for any slide object. PowerPoint’s Re-Order options for animations lets you play with their sequencing.