Wood burning stoves have flat fuel beds because wood burns better on a flat bed of ash, with air for combustion coming from above. Multi fuel stoves are fitted with grates because coal and smokeless fuels need to burn on top of a grate, with air entering from beneath the fire and the remainder of the fuel falling through the bars. Multifuel stoves can burn wood too as they are also designed to allow you to build up a bed of ash upon which to burn logs should you wish.