Many firms have bargaining power over their workers.
They under-pay their workers in a free market.
Minimum wages mean they correct this, and may hire more workers as a result.
-Availability of substitute labour
-Time frame
-Training required
Occupational licenses restrict the number of workers available to do the job, shifting labour supply curve left and pushing up wages.
-If labour is more expensive, employers want less of it.
-Workers are paid for the revenue they can make for the firm.
-Workers who are not skilled enough to make revenue above the minimum wage will not have a job.
Because workers have earned so much that they want to trade extra work for extra leisure (income effect > substitution effect)
Because if a firm provides workers with capital equipment to increase their productivity, there is no need to make them unemployed.
Because a higher wage will incentivise workers to work.
Households (individuals)
Trade unions typically restrict supply of labour (by insisting firms hire from the union), shifting labour supply curve left and pushing up wages.