Why does the lack of a magneto affect engine RPM?
Firstly, to understand the answer, we need to understand that the Cessna 152's and 172's run a 4 Cylinder, Horizontally-opposed Engine. Each Cylinder has 2 spark plugs, one on the top side of the cylinder head, and one on the bottom side. The spark plug ignites the fuel/air mixture that has been sucked into the engine, and causes a controlled burn to push the piston down the cylinder and turn the crankshaft in turn (hence turning the propeller as it is connected to the end of the crankshaft). 

Now, as I've stated, each cylinder has two spark plugs, one connected to the "left" magneto, and one connected to the "right". If we turn one magneto "off" (or ground it to be politically correct), only one spark plug in each cylinder will "fire" to ignite the fuel/air mixture. This causes the mixture to have a delayed and less effective burn, meaning that the piston does not get pushed down the cylinder as effectively, meaning the crankshaft will not rotate as fast, and hence, leads to a drop in the Propeller RPM (Revolutions per Minute).

We check the magnetos on the ground to check that:

A. They both work, and the engine won't die if we lose a magneto (we should be able to return to land on the remaining serviceable magneto)  

B. We are checking that there is no "Spark Plug Fouling". The fuel we use in aviation contains lead, and hence after a time of running the engine at low power settings on the ground, the lead in the fuel will accumulate on the spark plug head (instead of being burnt off when the engine is working under power), and cause the spark plug to let off a limited spark, or completely block the spark, hence again, making our engine less efficient.