Quick verdict
Email is the default because it is already there. It works until the request has more than a few moving parts.
If onboarding requires files, forms, access details, approvals, and follow-up, a portal is clearer for both sides.
Why email feels easy
It is fast to send. Clients know how to reply. There is no new system to explain.
That is why email is fine for one or two simple requests.
Why email gets expensive
The cost appears later. Attachments split across threads, clients answer half the questions, files arrive without context, and nobody is sure which version is final.
The agency ends up doing the organizing work manually.
Portal advantage
A portal makes the request visible. It shows what is complete, what is missing, and what needs revision. Instead of chasing the client with another thread, you point them back to the exact open items.