The Creepy Dolls?

It showed that the Marquis was more interested in gadgetry than his wife. It was kind of a loveless marriage. Another way that is shown is the early conversation between Christine and Lisette about having children. Christine's look at one point makes it seem like she knows she'll never have any. The Marquis's demonstrations of affection are only towards the mechanical devices that he's obsessed with - and briefly, with his mistress.

A few more possibilities (not necessarily mutually exclusive):

- Fantastical mechanical devices had been all the rage with almost everyone in the upper classes one or two centuries earlier. So they show that Cheyniest was "stuck in the past".

- It's hard to think of any other hobby that's more impractical or useless. So they emphasize how frivolous Cheyniest was from the point of view of the larger society.

- They're 100% predictable and they never talk back and never go out on strike. Thus they symbolize how unprepared to deal with real people Cheyniest was.

- Knowing how to collect and fix and maintain the dolls is a skill that doesn't seem to transfer to much of anything else. So although Cheyniest was an expert at dealing with the dolls, it didn't at all prepare him to become a political leader.