Steve Jobs says they want to expand their browser market share.
Other's say that it will possibly work as a trojan horse to get people to buy more Macs.
While the first one will no doubt happen almost immediately, the second one is highly unlikely.
There are three other reasons why Apple went this path.
Internet Explorer for Mac was always a bit shady. It was obvious that more resources went into the Windows product than the Mac product, and that was understandable. But Mac users always felt frustrated that many sites that were written to work for IE, never did work on the Mac version. Crucial things like online banking were generally hit or miss.
Although many alternative browsers started to appear on the Mac over time, it was no doubt one of Apple's greatest fears that MS would drop IE support and leave them without access to the most popular browser in the world. If other browsers started to disappear from the platform as a whole, that would put Apple in a very dangerous position.
That fear is what likely drove Apple to create their own browser. Safari was a nice iteration for Mac users as it was more Mac-like, and Apple was able to tie the browser closer to the operating system at the same time. In addition, Safari provided Apple with a platform for the future.
The problem still has been that not all sites work with Safari. A long-time Safari user myself, last year I switched over to Firefox simply due to compatibility issues with blogging platforms, banking, etc. On top of that Safari, updates were few and far between in comparison to other browsers out there. And even though Safari had many great features that I loved, it was also a system hog that sometimes brought my machine to a crawl...and if you over taxed it, it would sometimes crash with no session restoration.
So at first I was a bit puzzled that Apple would release Safari on Windows, which with all the Window's-based malware and security issues, would suck more developer time from releasing Mac updates and might also overshadow the positive attention with negative.
I think it is ridiculous to think that a Windows version of Mac software will create new switchers. It just isn't going to happen. What will likely happen however are three things:
1. As Safari continues to be downloaded and grow in market share, we'll see more and more developers of web sites that will test their code to see that it complies and works with Safari. Good for Mac users...good for Apple. I don't think it is impossible to see Apple jump another 5-10% in browser market share in the next 6 months.
2. It might save Quicktime, Apple's slowly dying web streaming technology. While Apple might debate the dying aspect (given it travels with every version of iTunes), almost no site outside of Apple uses Quicktime anymore (I rarely see it) and there is no Windows Media support (MS killed it). Not to mention that flash has made some serious inroads on Apple's once strong web technology. But with better Quicktime integration into the Windows-based browser, we might see a new push to recapture lost Quicktime market share.
3. iPhone, iPhone, iPhone. We know that Apple is releasing a Safari/webdev kit so developers can develop web apps for their new phone. Why limit this to Mac developers? The phone is obviously designed to appeal to Mac and Windows users, so to ensure development for the phone on the Windows side, they need a platform to build on. Safari will no doubt be the major component that ties the phone and iTunes together, and we'll likely see an explosion of web app development this fall after the phone is released. Windows support is crucial to their long-term phone strategy and that is especially important when it comes to browsing. Just look at all the sites that are popping up to work with the Wii...I have no doubt in my mind we'll see lean and mean Safari sites for the iPhone.
One bit of advice for Apple.
Windows users aren't going to tolerate once a year Safari updates and Mac users won't appreciate Safari for Windows diluting developer time away from the Mac version. If you plan to support both platforms, you need to ensure that you release timely updates with new features more often.