I don't think the goal is to be an alternative to Windows. There are many situations where you might want to run Windows apps but don't need everything Windows has to offer. Like others have mentioned, supporting legacy apps is one example of this. There is also research value in having a Windows-compatible OS which is open source.
There are some legacy industrial software we had to keep (for cost reasons) that run on ReactOS with little to no configuration changes that completely break in WINE or run glacially slow on compatibility mode on Windows 7.