After using Typora for a long time, I really don't think a separate preview is that necessary.
but when you're using a full range of complex features, it is.
and when something isn't displaying the way you intend, you often need to view the input _with_ the output, to see exactly what's causing the problem and to test possible solutions.
i also found that, while typora is good at showing what the output looks like in a browser-window which is sized exactly like the one you are typing in, it's less informative at showing what the content will look like in a .pdf or a .mobi or an .epub.
and, just for the sake of completion here, typora also became very unresponsive when i tested it with large files. again, not everyone will have that problem, but some users definitely will.
but i'll give typora another go, and see if it's improved since i last tried it...