A portal is a separate front door for the people you work for, rather than with. Clients sign in and see records that belong to them — and nothing else.
A workspace member sees a workspace: spaces, sheets, other people's work. That is right for colleagues and wrong for clients.
Portal users are different:
Your data stays in your sheets, as it already is. The portal is a lens onto it, scoped per customer. Nothing is duplicated, and nothing has to be exported and sent.
They do not appear in Settings → Users and are managed separately — which is also why they do not affect your bill.
Related: Build a form and collect responses · Roles and permissions · Seats: adding and removing people