Differences are geometry of the frame and components. The theory is that women will need a shorter top tube on a size frame that would be right for her in the seat tube length and angle. So when they shorten the top tube, they may make additional changes to the fork/head tube and rear triangle to let the bike have the same feel and handling as the standard version.
Advantages: narrower/short reach bars, women's saddle. If she needs a small frame size, it might be easier to find the small size in a WSD bike.
Disadvantages: it might not fit her. Get her a bike that will fit her. This may or not be a WSD bike. If she wants a women's specific saddle, that's an easy swap to make.