Preview page versus client page
Only users you have invited to your account have access to the platform. Normally you would not invite your client for such access. The pages you can access without an account, and are shared with clients, are the Preview page and the Client page.
This is the most important and most used page. This page shows the files you have uploaded and clients can leave feedback and approve the files. An example of a preview page can be found here.
A client page contains all the preview pages for that specific client. By sharing this URL with your client, your client will have an overview of all the campaigns or projects you are running for them. They could also use it as an archive with banner ads. An example of a client page can be found here.