Why use Simplenote for sync when you can sync .txt files using Dropbox or OneDrive?
For me, it's because Simplenote (or whichever client you use to access it) is actually an editing app in addition to being the sync app, and I quite like using the apps I have for it to do the actual writing.
I'm not thrilled with using Dropbox on my phone because (and maybe it's just that I'm not a super-experienced Dropbox user) I have to go into the Dropbox app, navigate through my folder system to get the file I want, open in and select in which app I want to open it, and then I get the file. I'm guessing there are ways to keep certain folders on your phone in sync with Dropbox, but it's not something I really ever got into since that's not how I use Dropbox. (I use it for other things, but not really text stuff.)
Right now, I "simply" open the SimpleNote app and I'm reading/editing an up-to-date, instantly-syncing version of the file right away.
It's the same thing (though not as cumbersome) on the laptop. Yes, I could have a plain text file in Dropbox that's easy to find and click on in Finder, but then I still need to choose which app to use to edit/view the file. And honestly, right now MetaNota is my app of preference as a quick text editor anyway (though as a result of this thread I'm giving the official Simplenote Mac app another go), so I feel like I'm skipping a step I'd otherwise have with Dropbox.
And, I also have the option of logging in to Simplenote online in any browser and editing right within the website, too. Accessing Simplenote online via the browser is also an editor, not just a filesystem. (It doesn't come up often, but being to access things through any old browser is a good option to have when you travel, for example, and sometimes want to use a hotel's business centre's computer for something.)
So, syncing .txt files when I don't really have a text editor I like any better than these clients just doesn't make a lot of sense for me. Of course, I understand why people like to have text files around and in their cloud/folder systems.
For my purposes, with it being more of a scratch pad, a place for rough drafts or random exerpts of writing, things to remember that aren't to-do list urgent etc., Simplenote serves me very well.