Given the german naming convention I expected this to be an SPG.

With a gun barrel that long at that point in the war I can imagine some real difficulties in meaningful concealment or even movement without getting hung-up. Using the hull as the basis for a mobile rocket system sim. to the postwar Raketenpanzerjager would have been more interesting if we're suspending credibility and playing the "what-if" game.

If the E-25 wasn't already a premium vehicle and only available in the one configuration, I'd suspect that this was a play to make a version of a vehicle from the World of Tanks MOBA. I don't know about other players, but when I see tanks with guns like these, like the /L100 guns researchable for some vehicles, I tend to refer to them as 'lolcannon' for their lack of real-world utility, only the game's lack of collision modeling for cannon barrels keeping them playable on many maps.