I could answer this question saying that it's better, stronger, easier: that would be false. The real reason is that there exists some PDF that use advanced graphics feature of PDF that are simply not correctly implemented in other free readers.

I prefer to use free applications, as much as possible. But technical value is sometimes a strong enough incentive to switch to proprietary software, such as Adobe Acrobat Reader. Of course, I will give examples to support my claims (pictures down there). I may be mistaken, but please feel free to correct me.

Before I start, let me state that I usually use xpdf for daily work: it is fast, it makes things easy, reload is as simple as pressing "r" or changing a page. But sometimes, xpdf does not do what I would like it to do.