Do you really need top coat?

Whether you really need 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 not washable, not durable, not UV resistant, and look flat and chalky, top coat provides true color, sheen, washability, and protection, so without top coat paint job fails quickly. For example, primer alone on exterior will degrade in months. 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 paint may be sufficient for low use areas. 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 top coat greatly extends wear. For art and crafts, top coat protects from dust and UV. So while you can skip top coat in some low wear temporary situations, for durable, washable, glossy, long lasting finish, top coat is really needed and highly recommended.