The hidden job market is a decentralized labour exchange. The labour market still exists. Workers still sell their time and their brains and their physical labour.
Employers still buy time and brains and physical labour.
The advantages to decentralization are obvious to both the employer and the aggressive job seekers. Employers have an objective way to measure in potential workers such desirable employee attributes as initiative, attitude, and persistence. Decentralizing the market for local jobs down to the level of individual employer and individual worker automatically screens out people lacking in critical traits, and employers offering attractive opportunities do not need to advertise to find good people, because good people find them. A decentralized labour exchange, a hidden job market, also works well for aggressive job seekers. It provides them the opportunity to demonstrate those intangible attributes to employers.