With so many stock photos easily found online that are free or very cheap, why would anyone choose to use a rights managed stock photo?
Let's say you just created a great web page for your company using a stock photo you bought online for a $1. On your way home from work you drive by a billboard and see the same image! The compelling nature of a photo is lost if it is used in too many places and for varying uses.
If you choose to pay more money for a rights managed photo it will be much more likely that you won't see that particular photo elsewhere, especially used in the same way that you want to use it.
Also, it may be more likely that a rights managed image was taken by a professional photographer because a site requiring a royalty fee monitors those images carefully. But, free stock photos can be taken by amateurs and that may not suit your purpose.