Sump pumps are strange machines. First, they have to sit low, and then they have to pump water up high against gravity. You would think that keeping the sump pump opening level or above the basement floor would make better sense. Actually, it does not. Here is why your basement needs to be graded on a downhill slope toward the sump pump prior to sump pump installation.
The same confounding force that the pump has to push up against vertically is the same one that helps the water to drain and collect in the sump pump drain. The basement floor is sloped downward to aid gravity with moving the water toward the drain. If the drain were level with the basement floor, the water would not bother to move toward the drain on its own; it has to be helped in that direction. Usually, the pump drain is placed where the most water tends to collect, and any water left will eventually evaporate.
Because...the Pump Needs Distance to Create Pressure
Try this little experiment. Cut a straw into two unequal parts, and use a third straw as the longer straw. Take turns with each straw, blowing air bubbles into a glass of water. What do you see? The shorter the straw, the lesser the pressure, and the shorter the distance the air pressure travels.
The same holds true for a sump pump. You have a motor and a pump that is attempting to suck up and push out the water that has collected in the drain hole. If you elevated the drain hole, not only would there be no water accumulating in the drain, but the pump would also not be able to create enough pressure to push the water against the force of gravity. This is why the floor slopes downward; first to allow water to collect in the drain, and second, to give the pump enough distance to create enough force.
If and when your basement should ever flood, you want to get the flood waters out of the way as fast as you can. Then the basement can begin drying up and the cleanup process can start. Do you know how hard it is to bail out a basement with no sump pump assisting? It is not easy at all. At least with a sump pump installed on a downhill slope, the flood water you are trying to remove by hand is aided by the water flowing into the sump pump drain and pumped out to the outside away from the house.