Email is a poor way to collect tax documents: attachments get lost in threads, files arrive with unhelpful names, and sensitive information such as Social Security numbers ends up sitting in inboxes. A portal gives each client one place to see the checklist of what's needed and to upload it.
Portals differ mainly in how clients get in. Some require every client to create an account and password; others use a secure link that expires, which is easier for clients who log in once a year.
Whatever the portal, the firm still needs a process around it: a clear checklist per client, due dates, and reminders for what's still missing.
Free resource: Client tax document checklist.