155mm Artillery launches rounds down range! What is the range of 155mm H...

This is the US Army's 155mm M109a7 self propelled howitzer.
It too has a standard range of 24km and can fire the Excalibur out to 40km. It takes 1 to 2 minutes to bring into action and fires 5 rpm.

It weighs 25 tons and while it has good cross country mobility it has poor strategic mobility. It needs a heavy lift plane or a specializ heavy mover truck.
First lets see why a Howitzer in general is a desirable weapon. It is because it has:

Long range: 20 to 50 km, depending on the ammunition and the length of barrel (caliber). In comparison, the range of tanks and mortars is limited to around 5 km.
Indirect fire capability: A Howitzer relies on the shell trajectory, not on direct line of sight. It can engage a target on the other side of a mountain, similar to a person lobbing a ball accross a high wall. A mortar has similar capability, but does not have long range. A tank is capable of only a direct line of fire - similar to a rifle.
Elevation: Tanks have a elevation of +25 degrees to -10 degrees. Howitzers have elevation of +70 degrees to -5 degrees. A Howitzer will be able to hit a target on top of a mountain, while a tank will not.
The only weapon that comes close to a Howitzers capability in terms of range and indirect fire capability is the MLRS - Multi Launch Rocket System. It delivers a high initial rate of fire and has ranges beyond 50 km.

On its own, a single MLRS unit cannot match the accuracy and a sustained rate of fire of a single Howitzer. However, modern MLRS with the support of fire control computers, radars, GPS/inertial navigation systems and laser target designators can match or exceed the accuracy and rate of fire of Howitzers.