It doesn't matter if your symlink points to a file that's inside or outside your Dropbox folder.
The symlink in the original computer is also the only thing that keeps both folders connected.
If you change only one of the folders in the web interface, when the original computer syncs it will replicate the change to the other folder.
The big cloud storage services — Dropbox, Google Drive, Microsoft One Drive, and others — all have the same problem.
They can only synchronize folders inside your cloud storage folder.
Since the Dropbox client needs to remove the "folder" Note that Dropbox is not a backup utility, but a file synchronization utility. If you install Dropbox in a new machine, you will have to manually recreate all the symlinks.
We made the decision to handle symlinks transparently, so that the complete file tree would be exactly the same on all operating systems and on our servers, as presented by the file managers. N., Dropbox forums moderator Since symlinks are not preserved, avoid using Dropbox as a backup utility. Given all these gotchas, it's better to avoid using symlinks inside your Dropbox folder until they add real support for them in the server.
If you format that computer, the connection is gone and each folder is now really independent of each other. Never remove, via web or mobile interface, a folder that's in fact a symlink in the original computer.
Both the symlink and the original folder contents will be removed.
To include your Desktop folder into Dropbox, just make the Note that this practice will become obsolete when (if? Say you just made the symlink folder in your Dropbox account.