Fully furnished means the flat comes with all furnishing like A/C, beds, cupboards, table, chairs, fridge, Cooking range, washing machine etc... semi- furnished means only the kitchen is furnished and A/C provided. The rest is the tenant's... Unfurnished means flat with bare walls... If someone already has furnitures he will usually prefer to have an unfurnished flat... Also many others prefer to furnish the house in their own way... However some will go for the fully furnished flats to avoid the responsibility of buying any furniture... Hope you have an idea now...

All the best...