When a person applies for a work visa, in most cases, as part of the application the employer has to show to Immigration New Zealand that it has made a genuine attempt to find New Zealanders, and that it has been unable to (this is called "labour market check"). This means that each time a prospective employee is applying for a work visa, the employer must have already advertised the position sufficiently. If Immigration New Zealand determines that the recruiting efforts have not been sufficient, then the application could be declined on that basis.
Having an Approval in Principle in place could benefit employers, as the time spent in advertising the position will be cut down, and the processing time of the application by Immigration New Zealand is likely to be quicker. An Approval in Principle will specify what position is offered, how many positions are available, what are the terms of employment (e.g. rate of pay, minimum requirement, etc), and its validity (usually 6 months).
With the Approval in Principle in place, once the employer has found someone who fits the criteria, then the visa application could be lodged straight away without having to muddle around. Please note, however, that an Approval in Principle does not replace the process for applying for individual work visas, nor guarantees a successful outcome, as the work visa applications lodged under the Approval in Principle will still be subject to other assessment criteria such as medical, character, background/qualification etc of the applicant.