Speculation wise, setting up a legitimate marketplace for paid mods in games where developers have given their consent for these transactions to take place is Valve's venture/experiment into further empowering content creators.

Valve has been pushing for giving the necessary tools and marketplace for content creators for a while now. For example, Valve has so far paid out $57 million to TF2, Dota 2, and CS:GO workshop artists, and they're designing Source 2 engine with increasing productivity of content creation. It's fairly established to Valve that selling user-created cosmetic through in-game stores is a viable method of selling what are would have normally been mods. The incentive for buying is there due to various reasons, such as for players they want to show off their cosmetics to friends and strangers and for creators they gain access to the audience of a few of the most played games in PC. Considering the success of this market, it appears surprising that the first large non-Valve game that supports paid mods isn't a multiplayer game with a huge player base but rather a single player game, but considering that Valve probably wants more data on other type of games it doesn't seem as surprising that Valve has decided to partner with Besthesda on selling paid mods.

Paid mods for single player games hasn't really been explored at a large level, at least not in the public eye as far as I know. What better test is there than seeing if paid mods can become a legitimate part of gaming if the large modding community and extremely popular Skyrim was combined with the popularity of Steam as a marketplace? I think a lot of companies will be looking at this very closely. If it fails, then paid modding will be less likely to take off in the near future. If it succeeds, many things can occur.

First, in paid mods the developer (for the case of selling paid mods through Steam) receives a part of the revenue. If developers believe that paid mods is a legitimate way of generating revenue for their company, they may be more inclined to embrace modding and develop proper modding tools. Now the quality of the tools will vary depending on various factors, including the support from the developers, popularity of the game, and competition of better modding tools from other developers, but the overall quality of modding tools for the most popular games should increase if paid mods become a serious business in order to attract the best modders.