It's primarily a file manager giving you access to everything found on your SD card and is very useful for managing media or for when you download things off of emails or the web. Astro can also show you a breakdown of what's taking up space on your SD card, be used as a task manager, and check md5sums which is handy if you're the custom ROM type.
Astro is basically a static app. It only really uses battery and runs when you are currently using it.