I don't personally think it's about the market getting big in itself, since the appeal of the medium is small enough that it's clearly never going to hold a candle to the major sellers on either Steam or GOG. However, GOG have already had some success as an outlet for Japanese games (Tokyo Xanadu was a chart-topper there despite being a fairy mediocre title), and visual novels have an overlapping market with such Japanese titles.
If I had to guess, then, part of the point is to be visibly welcoming to a certain demographic, while also indirectly snubbing Steam. It's poor "optics" with respect to some of the population, but if GOG are betting on the idea that those people (approving of censorship, and so forth) are more likely to be Steam-loyal than to share their no-DRM ideology, then it would be quite a strong way of boosting their presence as a freer and more consumer-friendly alternative, in the business of giving all customers their desired product without putting unnecessary obstacles in their way.
So, they get to increase their appeal to the Japanese-content-loving demographic, and also increase their appeal to the libertarian demographic (more or less their core), at the same time. Win-win for them, really.