I have heard technicians say that they improve the sound of some pianos, making them sound "bigger" than they are.
They are about 25% more expensive, require care in installation (because they break more easily), and are not suited to high-tension scales (because of their lower breaking strength).
Yes, you must match plain wire stainless strings with bass strings with a stainless core. I don't remember who makes those bass strings. And then you must leave a note for the next technician somewhere, so that they replace any broken strings with the same kind...
--Cy--
P.S. Here's Stuart's site:
https://www.stuartandsons.com/