Im not sure about the economia of Servia, but some political points:

1. Anything you gain is good
2. Weakening Russia right now is of high value
3. In the center of a lot of ex-urss
4. they have a serious communist party (not 100% sure its Servia, so maybe this one is wrong)

I think its kinda like in América Latina, the empire is taking the small countries in preparation to try to take Brasil. I wouldnt be surprised if Servia is the most politically organized (in terms of workers interest) country in the region, it makes you an important target for the empire