Unless you live under a rock, you've seen something built with fluid concrete. (Or, who knows? Perhaps the rock you're under is a chunk of fluid concrete.) Contractors and engineers often use it on the job because its thinner consistency makes it ideal for a variety of projects.
One benefit of using fluid concrete in construction is the ease with which it can be pumped into places trucks can't go. To push the thin mixture, builders pour it into the hopper of a specially-designed pump mounted on a truck or trailer. The pump moves the mixture through pipes made of rubber or steel that either run along the ground to move concrete horizontally or are attached to a mechanical arm (resembling the one on a cherry picker) to transport it vertically. Because fluid concrete is so thin, contractors can pump it great distances or heights without having to increase the pressure to keep it flowing as they would with thicker, standard concrete. You might compare it to the difference between sucking a soda through a straw and sucking a milkshake through a straw; the thinner beverage just takes less effort to drink.
Fluid concrete can also be useful for filling forms -- structures typically made of wood or steel that help the concrete hold its shape while it's wet. You've probably seen simple forms bordering sidewalks or driveways on construction sites; they're removed when the concrete hardens. Forms that are particularly narrow (like those for walls), are heavily reinforced (with steel mesh or rods known as rebar), or have embedded items (like pipes or bolts), may be filled with fluid concrete because it's able to seep into narrow spaces and hug impediments.
Using fluid concrete also reduces the work that has to be done at the construction site. For builders attempting to pour a level foundation for a building or other structure, fluid concrete is thin enough that it can pretty much level itself, much like the water in a swimming pool when it becomes very still. Time is also saved because there's no compaction process for fluid concrete. Standard concrete must be compacted using a special tool that consists of a small gas or electric motor connected to a short hose that vibrates on the end. The vibration removes air pockets that may have become trapped in the mix when it was poured. Fluid concrete doesn't have to be compacted because air isn't as likely to become trapped in the thin, soupy mixture.