The "Simulate Document" feature is available for Mac .xib file(s) and the "Simulate Document" feature was under Editor.
Is it possible to "Simulate Interface" in Xcode 6.0.1?
If it is gone, what is the initial purpose for having it at the first place and why it is gone?
Reference:
Interface Builder's “Simulate Interface” is gone in xcode 4.6?
The "Simulate Document" feature is gone because of new "Live Rendering" feature.
Live rendering. Interface Builder displays your custom objects at design time exactly as they appear when your app is run. When you update the code for your custom view, the Interface Builder design canvas updates automatically with the new look you just entered in the source editor, with no need to build and run. You can use the Interface Builder inspector to see properties automatically as well. Take advantage of new APIs that allow you to customize the behavior of custom controls on the Interface Builder canvas—for instance, you can load sample data on the fly.
For more information on live rendering, see Creating a Custom View That Renders in Interface Builder.
Reference:
https://developer.apple.com/library/ios/documentation/DeveloperTools/Conceptual/WhatsNewXcode/Articles/xcode_6_0.html:
xcode 6 has live preview of controls so simulate has no longer sense
