Why is "100% Japan-made Solid Capacitor" a selling point? [closed]

Well, there's two factors in play here, both to do with the dreaded capacitor plague that killed so many computers of the naughties stone dead.

Basically, electrolytic capacitors are cheap, have great capacitance per unit volume but are often kind of lacking in other respects, such as ESR. Some capacitors, made by Taiwanese companies used a flawed formula that ended up with those capacitors failing catastrophically. This was a specific formula, used over a period, and modern capacitors are likely to use fixed formulas that are more reliable.

The solid capacitors are a newer design using organic semiconductors or OSCON. They couple low ESR (which is good) and a wider temperature stability range (since the solid capacitors can't/won't freeze or boil). They're significantly better than the garden variety capacitors in most respects - to compare, the maximum temperature the average liquid capacitor can handle is about 85-95 degrees, while the solid caps handle in excess of a hundred. They also have better characteristics in general, but I doubt anything that most users would notice.

They don't fail the same way (catastrophic bursting) as electrolytics - though I can't find any literature on this outside this website which suggests they can silently fail by moisture infiltration resulting in increasing ESR, but that they've never seen it before

The fact that they are japanese? Not made by the lowest bidder, and often by the companies that developed the technology. These capacitors are made elsewhere too, and have been around a while.

At the end of the day, many motherboards that come with these capacitors also have other features to improve reliability, and arn't made of parts that are picked mainly for cheapness.

It seems Taiwanese and Chinese sources of capacitors may have been the ones involved in the capacitor plague. I'm not sure if only capacitors from those countries were the ones prematurely failing, but that could be a basis for the marketing claim.

The "solid" capacitors (mainly produced in Japan, but not necessarily Japanese) are purported to be of higher quality, with more refined electrolyte and with better sealing capacity than standard capacitors. This means that when (slightly) abused, or growing old, they are less prone to explode and/or leak electrolyte. In marketspeech, "less prone" becomes then "they won't leak. Ever!". Electrolyte-less capacitors will of course not leak as they've nothing that can leak -- but you can blow them just as easy as ordinary capacitors. There's no such thing as magic, alas.

In motherboards my opinion is that this not very relevant, for high-end motherboards are built with higher tolerances and quality control, and they won't experience capacitor leak for a very long time; and low-end motherboards will be bought (or should be bought) with a short life expectancy anyway. Seeing "japanese solid capacitor" touted as a sell point would make me think that they've been included in the project as marketing sugar, nothing more.