Why do we need to use the Portal for SRM 7.0 EHP2?

Hi my client wants to know WHY specifically we need to use a portal when implementing SRM 7.0 EHP2.

Our scenario is, extended classic, with MDM catalog, TREX, contract management, RFX (not bidding) back end is ECC 6.0 EHP ?.  We are not using e-sourcing (frictionless).

They want to avoid adding to the landscape if possible and are questioning the absolute necessity of using a portal in these circumstances.

Any light anyone can shed on the absolute need for a portal in our scenario I'd appreciate it!

The docs I can find, say mandatory, but not why mandatory.....

Thanks

Tracey