Hi @Acey Tseng, thanks for messaging the Community!
If you sign out of the Dropbox desktop application, then nothing will happen to the files in the Dropbox folder, meaning it doesn't automatically delete.
If you sign in to the same account again, and the Dropbox folder exists with the same files, the app will check for any updates and sync any new or edited files. Depending on your settings, it will either make them available offline, or just add new placeholders if new files were added on the site.
If you have any further queries, feel free to message back.