Tristan Louis has a point when he says "Apple is making sure that more developers ensure their sites work with Safari" and may be onto something in suggesting that "Apple is basically pushing Safari as a new platform" although that's more obscure.
While the majority of websites work well enough with just about any modern browser, plenty of them treat Internet Explorer as though it defined the standard, making use of techniques that IE supports, irrespective of whether they've been adopted by the World Wide Web Consortium, the ISO, or any other industry standards setting organization, and without regard to whether they exist in other browsers. Since doing so might fairly be characterized as complicity in restraint of trade, I'll call these BAD websites, and I'll get back to them in a bit.
That point about restraint of trade maybe needs further elaboration. You see, Internet Explorer is only being actively developed for the Windows platform. The most recent version available on any other platform, version 5.2.3 for Mac OS X, is two major versions behind what's now shipping with Windows. The only way to run the current version of IE is to first run Windows, which of course means that you must have a copy of Windows.
Now Microsoft is quite happy to sell you a copy of Windows, even one that you can run on a virtual machine alongside Mac OS X or Linux, so these days you don't actually need a dedicated Windows machine to get Internet Explorer running, but you still have the expense of at least the basic version of Windows, and the bottom line is that, if you need Internet Explorer for compatibility with certain websites, Windows is necessary and any other platform is insufficient; you can load Windows onto your Mac, but you can't use a Mac instead. So what are you going to do?
Of course, Microsoft hasn't forced anyone to support Internet Explorer exclusively; that's the choice of web developers and the companies and agencies they work for. It's ostensibly about cost control, although I sincerely doubt that it's any cheaper to develop a website that will only work with Internet Explorer than to develop one that complies with broadly supported standards.
Admittedly, it has begun to become common for some of these websites to also support the number two browser, FireFox. Unfortunately for Apple, Safari is number three, and there's where you'll find the main point behind Safari for Windows. If Safari represented ten or fifteen percent of the market, instead of five, it would be harder to ignore, and as web developers ceased to ignore it the need for Internet Explorer would go away.
In his keynote, Steve Jobs showed a pie chart depicting the market shares of IE, FireFox, Safari, and all others, followed by a chart where FireFox and the others had been replaced by Safari, and said that's what Apple would like to see. I don't believe that; I think they're not so concerned about FireFox, which is more standards-oriented than IE and is being actively developed for Mac OS X, and would rather take a big market share bite out of IE. Pressing forward with the development of Safari for Windows is the most direct path to that goal. That second chart, showing Safari swallowing up FireFox without taking anything away from IE, is as close as you'll ever see Steve Jobs come to kowtowing to Microsoft. Why he felt it necessary to do so is a bit mysterious. Perhaps it has something to do with Microsoft's ability to break application programs by altering the platform they have to run on, something they are notoriously reputed to have done on at least one previous occasion.
In retrospect, this was totally obvious, and Safari for Windows shouldn't have been a surprise.
As to Safari being a new platform, I think that misses the point that Apple really is a huge supporter of web standards. The web is the platform; Safari is an implementation of it. Sure, Apple has done a few things outside of the standards, but they've then pushed for the adoption of those innovations, rather than standing aloof from the standards process.