The main purpose I believe was an attempt to remove a calibre from their inventory. Before the conflict in Syria they seemed to be in the process of removing the D-30 towed guns and also withdrawing the 2S1Ms from service... but they had so many parts and chassis I think they decided to replace the 122mm guns with the 120mm rifled gun/mortar of the Vena.
The 120mm gun mortar can fire shells with variable charge propellent loads to a max range of 13km... which is not really that much short of the 15km or so the D-30 and 2S1 122mm guns can manage, and the 21.8kg HE shells are not that much more effective than the 120mm shells that are 19.8kgs that the 120mm gun the Vena carries.
The 120mm gun /mortar can fire the Gran 120mm laser guided mortar bomb and also the 122mm Kitilov round fired by the 122mm gun, but it can also fire the entire range of 120mm mortar bombs too.
The reduced recoil and higher rate of fire of the 120mm weapon means it is a good solution for replacing the older system with a light and mobile vehicle they have available in numbers that can be used to support armoured forces.
(Keep in mind they have 120mm mortars, 122mm artillery pieces, and 125mm smoothbore tank guns in service... they are taking out the 100mm rifled tank guns of the T-54/55, and the 115mm smoothbore anti tank guns, though they are keeping the 100mm rifled medium pressure artillery guns of the BMP_3, and the 100mm smoothbore guns of the MT-12 towed AT guns...)
Syrian experience has shown that towed guns are also rather useful too however... the D-30 being a particularly good gun. Slower to set up than foreign equivalents, but able to fire in any direction without having to dig it out and move it... it can fire in 360 degrees without moving it.
In mountain combat with guided shells it probably could take out any enemy vehicle simply because a diving top attack missile bypasses the heaviest armour on any tank and goes for the thin upper armour and naturally evades most APS systems...
I suspect several new sensors as well were added and the Hosta was also included in the overall Russian C4ISR architecture.
Indeed... they have become rather potent vehicles that don't have to be lined up in a row to fire on a target... and their high gun elevation and variable propellent charge capacity makes them much more flexible than a tank gun.
For those not familiar a mortar is often very effective because it comes down nearly vertically so the walls of the shell with all the fragments create a nice even radial pattern of death, whereas a higher velocity guns round would come in at a shallow angle so a quarter of the fragments will go into the ground and another quarter go up into the air while the two side quarters go sideways so if you are in front or behind the shell you might get to live.
The other problem is that in the mountains targets over the hill can't be hit with a gun because to shoot them the high velocity of the round means once it goes over the hill it keeps going for a very long distance... tank guns don't have the elevation to fire up high to lob rounds over hills or rocks and even if they did a full power propellent charge means they would go up to enormous heights and spend a long time in the air all the while being pushed off trajectory by the wind at the various altitudes.
A mortar or gun mortar in this case has propellent bags like a howitzer so a reduced propellent charge makes the trajectory steeper so it will drop over the hill and land on the reverse slope... you can angle the barrel up steeply and reduce the charge to hit close targets with a short time of flight of the round making it much more accurate... and if you run out of shells local 120mm mortars can supply ammo too.
I would say get a blimp too that you can tether in position to provide radar coverage and observation in the area 24/7, and can be used as a radio repeater so your troops can use it to provide excellent radio communications (add a cell net too for cell phone comms).
Cheaper than flying drones or AWACS type aircraft over the area all the time.
Operate it out of small arms range of course... it will be targeted....