When you buy furniture, it comes two ways - assembled or flat packed. The perception of assembled or flat pack furniture is different for everyone. It's worth noting that neither format determines the quality of the piece. The fit, finish and materials used dictate the quality. Assembled furniture can be made with inferior quality materials and flat pack furniture can be made with top quality materials. There is a huge variation available on the market.
Furniture typically needs some assembly - whether that is done at the factory or by you. If you enjoy putting furniture together, you can typically get a lower price for the same quality.
Flat pack furniture is also great for compact spaces because it is easier to transport and get into small stairwells, lifts etc. You can then put the furniture together once it is in the space.
Lastly, flat pack furniture is less prone to damage because it hasn't been assembled. Most damage during transit occurs in the joints and corners so flat pack cartons usually fare better.