Not much, except being open, so WD can produce them without paying anyone royalties.
Unless they pass the savings on to their customers, that is. And even then, I am not so sure. Shaving a couple of cents of the price of a disk drive does not seem like a big deal to me.
Their stockholders might care, though.
This move, if it works well for WD, could lead to more attention being paid to a more open competitor to ARM, which would provide some competition and put downward pressure on ARM pricing. That, in turn, could have some potentially interesting second-order effects.
But yeah, if you only care about consumer prices and visible features, this is probably pretty boring stuff.
And if it's the latter - would WD buying a couple of billion chips a year have any effect on prices?
And now that you mention it - a company like WD announcing they will use RISC-V in their disks means they are serious about this, which in turn might make it easier for other to consider RISC-V a serious option.
I am very excited about RISC-V in theory, but unless somebody builds a "Raspberry-V", so to speak, it will probably be a long time before I get to play with one of these. I also think a high-performance implementation of RISC-V could make for an attractive component of a desktop machine / workstation. The Talos Raptor / II seems to be a sweet machine, but it is totally outside my budget. A less-high-end machine built around a RISC-V might change the equation.