I don't see the point then of using Dropbox exclusively in such fashion. All cloud storage services used like that are interchangeable.
I know there are dedicated backup services like Backblaze, however occasionally I also need to share a large file or two with someone or need to offer a public download. It's often enough that I don't want to use wetransfer or similar services. Even more often I need a service that syncs data and settings of various iOS apps. While iCloud is fine, Dropbox is widely supported on iOS. I basically need a general purpose web storage and Dropbox seems to be the only one that handles all these use cases fine.
I've always been a bit scattered across multiple cloud service, for various reasons, so I opted to use software that could sync with multiple and avoid installing so many clients.
That said, Dropbox's other maybe-not-quite "killer feature" is that it's exclusively intergrated into a number of applications for settings and data backup/syncing. In those cases, Dropbox's purpose is interchangable, but in practice it's not.