What a branded portal does well
Client portals collect documents, requests, signatures and messages in a secure place. Branding them reassures clients that they are dealing with your firm and not a random website. For tax-season work, where clients mostly upload and sign, a portal can be the only surface they touch.
Portals rarely show the client anything about their own finances, though. They move information toward the firm, and the finished work often goes back as a PDF or an email attachment rather than something the client can explore. The client receives the output but never the living record behind it.