Preferred access point - some queries

Apols if I have missed any previous explanation of this, but I am wondering
what reason/s lie behind RDA continuing the concept of "a preferred access
point" in cases of multiple authors for a work.

Is the reason primarily to accommodate the contingencies of MARC-based
cataloguing? Are there other reasons such as data exchange and
identification or other?

What difference/s does "a preferred access point" make in online databases
and user interfaces?

Thanks
Neil Godfrey