With no minimum wage, an employer who offers the absolute worst in predictability would have to offer a higher wage than the same sort of work with predictability-as-a-benefit. Similarly, an employer who offers lots of employee-friendly flexibility – you can always leave early, no penalties for refusing an offered shift, etc – could pay less.
People then get compensated for what they care most about, not what a legislature has decreed as the only acceptable wage rate. They'd also tend to sort into the workplaces that best fit their circumstances. Social surplus is maximized.
A wage floor prevents these sorts of adjustments, because it creates a surplus of hours-offered at the higher wage, while eliminating other hours-demanded. (For example, it's no longer economical to have 3 registers open, or to check on the restrooms hourly, or to bring customers their own orders as opposed to having them picked up, or to have someone wandering the store floor for issues/questions.) Thus the hours available can be rationed out as an inducement for other, harder-to-regulate concessions.
So while the wage floor makes employers (who continue to employ) pay more, in that one dimension, it strengthens their powers of control/coercion is most others.
The absolute best system for employees is tight labor markets, where if anything about the job is bad – wage, hours, respect – they can walk across the street for another job-on-offer at about the same wage with a better mix of everything. That best system is sabotaged by lots of well-intended restrictions on employment relationships, so we only see it occasionally, during transient booms and in boom regions (like, recently, North Dakota).