Sometimes a single mortar tube doesn't quite do it. That's where things like the Nebelwerfer come in (it's from WWII. Also, makes a good addition to a snowball fight. I.E. - "Every good snowball fight needs some werfed Nebels."). In the grim dark of the 41st millennium, that sort of technology continues on with the Rapier Quad Mortar. Also, sometimes a storm bolter just isn't good enough and you want your Rhino or other Space Marine tank to get some more firepower. As such, you can get some different pintle-mounted weapons on there.
With both of these, Forge World has your back. They are now taking orders for both of these sets. They will begin to ship out next week. Beware the enemies of man, some firepower is headed your way.