Do you really need a top coat?

Do you really need a top coat depends on surface and desired durability, but generally yes, top coat is needed for long lasting professional finish. For interior and exterior house walls, you need top coat because primer and undercoat alone are flat, porous, not washable, not UV resistant, and will scuff and stain easily and degrade quickly, especially exterior primer fails within months in sun, top coat provides true color, sheen, washability, and protection, so without top coat paint job fails early. For painted wood furniture and cabinets, you need top coat like polyurethane if you want extra protection against scratches and water, though high quality enamel may be sufficient for low use. For automotive painting, you need clear top coat over base color, as base coat alone has no gloss or UV protection and fades quickly. For nail polish, you don't strictly need top coat, but without it color chips within 2 to 3 days and has less shine, with top coat it lasts week or more. So while you can skip top coat in temporary low wear situations, for durable, washable, glossy, long lasting finish, top coat is really needed and highly recommended to protect underlying layers and provide final appearance.