If folks are going to cool RAM and MoBos, and anything for that matter considering the trend in CPU / GPU efficiencies, it's hard to make a "why" argument against PSUs. I do have one issue ... I can't see the block. If there was a drip, you'd never know. Not that the block couldn't be produced with no seams internal to the unit.
The "why" also includes the the same reason as anything else that's water cooled.... 1) it allows greater performance ... to get more out of the unit (takes it from 1200 to 1400 kw,) a 17% per increase. The "why" also includes dead silence. With today's more efficient designs, you often hit the voltage wall before the heat wall, so water cooling loses it's primary "raison d'être" .... if silence isn't important to you, in many cases, that's reason enough.
If "why" is on the table, we have the phenomenon of CLCs to explain ..... I have yet to understand the "why" behind those. There's also the why of "cause it's there" and "because we can". As for not much waste heat ... ? It's more heat than an i5 / i7 CPU generates when overclocked. At 100% load, the efficiency of a platinum unit is 90%. 10% of that is 140 watts is more than an i7 OC'd.
All that being said, this is a "proof of concept" design. Industry has long done these flagship models not so much for what is gained from the sale of the product but as a means to a) show the press and public what the company *can* accomplish and b) as a learning process and technology driver because of what they gain from the design and testing processes. First you have to demonstrate what can be done, then you just have to figure out how to make it cheaper. Much the same in any industry just as the technology demonstrated in the Dodge Hellcat will eventually trickle down to affordable models.