I would guess that this feature doesn't exist because OS X assumes you are using windows at the size they need to be anyway, not fullscreen like in Windows. And if this is the case, you can just PUT them side by side. Let's face it, if they are small enough to BE side by side, you can just put them side by side, and if they are too big to be side by side, you 'Apple+`' between them.
OS X is different in this repect, and the document based metaphor is IMO better.
But with documents you can esily put them side by side. So OSX doesn't fully implement the document based metaphor, if you think about it. We put documents side by side all the time with taking notes from a textbook, and even in our interface with a computer it is often taking information from a paper format and converting it to digital (with text or numerical based tasks anyway). We don't all have the luxury of a 30" ACD, so an easy way to put them side by side so you can transcribe/summarise without reverting to paper would be useful.
Tiling is the computer equivalent of spreading your documents out on the desk, albeit with less space, and is a common enough task in the physical world. I'm surprised that there is no simple feature to enable me to do that. If I'm taking notes from a website, or a pdf file, I'd like to be able to simply put them side by side, in much the same way as my notepad sits beside a textbook.
Sure it can be done by resizing windows, but that isn't as easy in OSX because you can only resize from one corner.
It's not a deal breaker, but if the OS claims to be easy to use, there should be a better solution
* Before I get attacked again, I am in general very happy with OSX, and find the fact that the OS generally dosesn't get in my way improves my productivity. I'd just like this little extra.
**So there be no doubt, it's not a "must have" feature, it's a "would like to have" feature
***Couldn't agree with you more, boxlight