Why do Ubuntu releases use names and not just numbers?

As the title says - why also use names and not just version numbers?

I guess reading this will answer your question

MarkShuttleworth said the following with regard to where the naming scheme originally came from:

So, what's with the "Funky Fairy" naming system? Many sensible people have wondered why we chose this naming scheme. It came about as a joke on a ferry between Circular Quay and somewhere else, in Sydney, Australia:

lifeless: how long before we make a first release?
sabdfl: it would need to be punchy. six months max.
lifeless: six months! thats not a lot of time for polish.
sabdfl: so we'll have to nickname it the warty warthog release.

And voila, the name stuck. The first mailing list for the Ubuntu team was called "warthogs", and we used to hang out on #warthogs on irc.freenode.net. For subsequent releases we wanted to stick with the "hog" names, so we had Hoary Hedgehog, and Grumpy Groundhog. But "Grumpy" just didn't sound right, for a release that was looking really good, and had fantastic community participation. So we looked around and came up with "Breezy Badger". We will still use "Grumpy Groundhog", but those plans are still a surprise to be announced... For those of you who think the chosen names could be improved, you might be relieved to know that the "Breezy Badger" was originally going to be the "Bendy Badger" (I still think that rocked). There were others... For all of our sanity we are going to try to keep these names alphabetical after Breezy. We might skip a few letters, and we'll have to wrap eventually. But the naming convention is here for a while longer, at least. The possibilities are endless. Gregarious Gnu? Antsy Aardvark? Phlegmatic Pheasant? You send 'em, we'll consider 'em.

lifeless is Robert Collins. sabdfl is Mark Shuttleworth.

Source: https://wiki.ubuntu.com/DevelopmentCodeNames

But they do use numbers for the distros 8.04, 9.04 ,10.04 ,11.04 ,12.04 ,12.10 etc .

The animal names are only used as development code names for the next release, if you look at all the Ubuntu marketing material (such as the web site) you'll notice it very rarely, if ever mentions the animal name of an Ubuntu release.