But with high unemployment you can't afford to be fired, because you are unlikely to get a new job. On the other hand, with low unemployment, workers can take more time to do tasks, refuse extra assignments, demand better pay, and strike to get it. The threat of being fired is not so powerful since new jobs are readily available and now it is the employer who will have trouble getting new workers. In capitalism, there is a constant battle over how much work will be done at what wages. Unemployment reduces all workers' bargaining power by reducing worker options and strengthening capitalist threats.
Full employment has the reverse effect.
For these reasons capitalists won't eliminate unemployment and, when it drops on its own, in due course they will intentionally induce new unemployment to reestablish their relative power. To win reduced unemployment, and particularly to win institutionalized full employment, workers must exert collective power. Because this gain would dramatically increase workers' bargaining power, it must be taken, not negotiated or begged. Moreover, the same can be said for welfare programs, unemployment benefits, increased minimum wages, a shorter work week, and even pay increases. Each of these gains directly redistributes wealth from capitalists to workers, and, more important, reduces the cost to workers of being fired thereby reducing workers' fear of pink slips and strengthening workers in their constant battle with capital.