Marked is the kind of application that made Unix great: it does a handful of things well, and thus works well as part of a larger tool chain, which in this case includes one's text editor, file browser, and perhaps other programs for viewing output if one exports to RTF, HTML, or PDF.
All the other apps directed toward Markdown usage are basically trying to be an IDE for a .md format that doesn't need that AT ALL: they're basically Rich Text input fields with editing widgets of varying complexity, and they save to a plain-text .md file.
These other editors are doing precisely the wrong thing for people who already LIKE their plain-text editor of choice, and want to work in that editor as much as possible. If I'm working in Sublime Text or TextMate or BBEdit, I just want to be able to compile the .md file and preview it easily. That's it. Once it's where I want it, then I might want to save it as a real HTML file, or as a PDF.
Or, working in the other direction, let's say I want to preview an existing .md file from the Finder (or, in my case, Path Finder). Marked comes with services that let you preview from the context menu or with a hotkey. Then, if you look at it in Marked and decide you need to edit it, you can hit cmd-E, and Marked will pop the file open in your editor of choice, which gives you a robust set of editing tools, rather than the stripped-down offerings of the dedicated Markdown editors.
That's pretty much all Marked does. It supports the *correct* Markdown workflow for people who already use a dedicated text editor as the core of their development, regardless of language. (I.e., most people who write code.)
If there's any problem with Marked, it's that the appearance of Marked's preview only changes when the source file is saved. This is how it was for me, when I started learning to code HTML in Xemacs back in the mid-90s, so it doesn't seem like a burden. But I gather there are people who really demand real-time updating. This won't do that.