Flash IS useful for animal photos, for a single reason - to get a catch-light in animal's eyes (that spark we all look for ). If you're lucky and the sun is behind you and the animal is looking at you, flash isn't needed. It's quiet likely that you'll use Aperture Priority (Av) for the shots, so Canon turns flash into Fill Flash at that mode (i.e. it adds light to existing exposure, instead of becoming primary source of light). This will soften shadows and create catch-lights. The flash you're looking at - 580EX - even has a panel that can be pulled out for that purpose. When you point the flash up, and with the panel pulled out, you get that 'catch-light' effect without the flash actually adding much light to the photo. To use flash in Program Mode where it can become main source of light - is frankly not useful for animal photos, though it could marginally be used for flowers.
All that above said, I usually do not use it - its too bulky and can scare off some animals. But I have same model you're looking at - and its an awesome flash, for the few times I use it.
And no - a light meter is not needed for flash photography OUTSIDE the studio (since you'll likely use Fill Flash most of the time). Not to say a light meter isn't useful (I have one).
A light meter should be above external flash on your list of gear to get. Specifically one that does incidence readings rather than just reflective light readings. In order to make best use of one or more external flashes, you're really going to want a proper meter.
OTOH, if you're doing studio work, you may find that experience and a histogram are good enough.
Flash is useful for lots of things. Portrait, wedding, macro, product, journalism etc.
Nature is just really where it isn't super useful. In theory you could light up a landscape with flashes but you would need some SERIOUS flash power for that. Small speed lights won't offer much. You could get a powerful flash light and light paint a landscape though. (very time consuming and difficult but produces very cool effects)
And like I said, flashes with wild animals usually aren't that useful. In the case of most wild animals you don't want to get close enough for a flash to be helpful anyway.