309 movement is used to transfer material to material - subject to both materials have same Unit of measurement. These transactions are useful handling with mostly liquefied materials considering mostly when you are mixing both.
When you transfer with 309 movement, for one material document with 2 line items created and similar accounting entries also.
Where as MMAM you are directly forcing system to change material type, where system throws error if Account determination is different between both types, stocks exists in current, any open reservations pending, account/value updation differs etc.