Why are there multiple receptacles by amperage?
I'm not really sure exactly what your question is in reference to the multiple room examples you give. Without referencing the exact NEC sections, it basically allows 15amp circuits in living spaces and bedrooms, a minimum of two 20amp circuits in kitchens with home runs (single purpose circuit) for certain Higher draw appliances such as refrigerators, dryers, water heaters, etc.  It has always been working practice to split up some lighting from receptacles and also use multiple circuits to separate higher use rooms such as a living room from bedrooms.   There are limits to the number of receptacles on a branch circuit, but I don't have the reference at hand.

As far as the different shapes of receptacles, 15amp, 20 amp, 30amp etc. This is quite obvious in the fact that the male plug on appliances is slaved to it's draw of amps. These different shapes assure that a machine that draws 17amps cannot be plugged into a 15amp feeder. Granted, one could put several 6 amp devises on one 15amp and overload it, but that is what the circuit breaker is for.  Under normal and historical uses in residential structures, it would be rare to easily overload a circuit with typical household items, such as lamps, stereos, fans etc. if someone is going to use multiple high draw appliances, then common sense has to kick in and design the circuit plan to accommodate the anticipated load. When i design a house plan, I always use a minimum of a 20 circuit panel(most of the time a 40)  and separate branch circuits to each room, basement and garage, as well as a few for lighting or outdoor usage.

I don't know if this really answers your question, but that's all I got for ya!