One private link where your client approves proposals, signs contracts and pays invoices. Your branding, no client login, nothing to install.
The portal is the answer to the question every client eventually asks: "where do I find that thing you sent me?" One link, per client, holding everything for the work you are doing together. Proposals to approve, contracts to sign, invoices to pay, files, and the conversation.
The portal opens from a private link. Your client does not create an account, does not choose a password, and does not get a "verify your email" message before they can look at an invoice.
This is a real design decision with a real trade-off, so it is worth being straight about it. A client portal that requires a login is more locked down; it also goes unused, because the person who needs to approve your proposal is not going to sign up for a tool to do it. Adoption is the whole value of a portal, and a portal nobody opens protects nothing.
On the Studio and Agency plans the portal carries your logo and colours, set per client, and can run on your own domain. Your client sees your studio. For an agency that is not vanity: a client who never learns the name of your back-office tool cannot go and hire it directly.
A freelancer uses the portal to stop losing links. An agency uses it to stop being the router between a client and four people internally. Everything the client needs is in the same place, which means the account manager is not forwarding PDFs, and the question about invoice three is attached to invoice three rather than sitting in somebody's personal inbox.