There are mainly two. Firstly ReactOS is open source. Secondly ReactOS is Free. Also Windows (especially the newer versions) are known to monitor all your activity by default. So if you're concerned about your privacy or just don't want to share any personal info, we promise (and you can check our source code) that we don't track any of your data.
So yeah, they are (at least were) monitoring not only what sites you went to, but also what you typed into them.
On top of that, I have several old (XP-era) laptops which still boot, still can play my older games and get on the Internet, etc. They're not really capable of running even Win7. ReactOS allows me to run a newer OS on those old IBM battleaxes.
Frankly, I'm only really worried about malware somehow sneaking aboard the thing, which is why it's on-network in the first place. I don't want people plugging flash drives into it. And fortunately, there are still antivirus products that support Windows XP, which keeps the riff-raff out.
Having said that, ReactOS still needs quite a bit of work to cover that niche, in my opinion.
https://www.catalog.update.microsoft.com/Search.aspx?q=KB936...