The bed should be completely horizontal. If not, the first layer has to struggle with different gap heights between nozzle and bed. This results in having areas with too much material and too less material. The printed part might be fall from the bed or show wrapping.

The bed should also be completely flat.

Completely means significantly less than the first layer height. Therefore the first layer might be printed higher than the others.

Automatic bed leveling can be done to have the printer adjusting material flow and correct a not completely leveled and flat bed.

Printer firmware as Marlin can deal with not ideal beds and correct the material flow accordingly.