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These are an elegant solution to a problem we struggled with a lot in the early days -- all message passing is naturally built on buffers holding contiguous data
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However often (usually) the data is not stored contiguously. One can address this with a set of small MPI_SEND commands but we want messages to be as big as possible as latency is so high
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One can copy all the data elements into a single buffer and transmit this but this is tedious for the user and not very efficient
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It has extra memory to memory copies which are often quite slow
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So derived datatypes can be used to set up arbitary memory templates with variable offsets and primitive datatypes. Derived datatypes can then be used in "ordinary" MPI calls in place of primitive datatypes MPI_REAL MPI_FLOAT etc.
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