There is an area in the Ukraine which the separatists are seeking. The whole area they call "New Russia" [Novoi-Russia] is larger than just the two provinces that they have taken control of. Part of the reason I think they are seeking this new area is that the actual areas they already control have little strategic value to Russia. But the new area the rebels are seeking has immense value to Russia because it contains the backbone of the Russian space program and manufacturing of military hardware.
That whole strip of land starting at Kharkiv and all the way down to the sea is one vast military industrial complex of immense strategic value to Russia.
[I am not saying Russia does not produce its own stuff. Rather that industry was purposely divided by Stalin so that one part of a plane would be produced in one region and another part in another region--so that all of the USSR would be dependent on every other region. This means that a significant part of Russia replacement parts and rocket parts is still being produced in the Ukraine. So the fact that business is down in the Ukraine is a good thing. It means they are no longer supplying the Russian military. I can imagine the reason is they don't like being shot at. You won't see this information about Russian weapons because anything made in the USSR proper they won't tell you where it is made. Only if it is made in satellites of the USSR do they name the country of origin.]

Just to give one example to build the kind of aircraft carrier that Russia wants to build is not possible in any existing Russian facilities. The building of Russian rockets and aircraft is largely done on Ukrainian soil. This was never a problem  because the work was simply done in the Ukraine an sent to Russia. Nowadays this arrangement has become strained. It is hard for me to imagine that separatists will see this gold mine of a preexisting military industrial area right across its border and not want to take it.
That is to say that they are probably hoping to expand and take that area. But it also seems clear to me that the Ukrainian people in those areas will not hand over that territory peacefully. So if the separatists want that area they will have to take it by force.
And the separatist are definitely getting Russian military hardware like the anti tank PTRS-41.