The reasoning is that many studies show that people that have not worked for a while have a difficult time being hired again.
There are different ideas, but, I suppose that you could see it like a training program too.
Also, there are some supposed psychological benefits to work. And there is always something to do at the local community level, so it would not be totally unproductive work.
The incentive is in the reward you get for performing well.
Also I don't think it's necessary that a job guarantee make an employee immune from being fired. I'd imagine a job guarantee would have all kinds of jobs ranging from relatively simple (eg. cleaning) to more complex (eg. science). If you're in a more demanding department and underperforming, you could get fired and have to apply to another job in another department.
For example, the military is sort of like a job guarantee in the sense that anyone meeting the physical bar is effectively hired. But any military member can get discharged.
That becomes an interesting dynamic expecially between universal guaranteed income and universal guaranteed employment.
But the original question was how to provide incentive. Assuming able-bodied persons must provide some work, opportunities for advancement, more pay, or more favourable environment, and the prospect of less attractive work, remain. Military and similar environments provide models, e.g., latrine duty.
Most people would prefer some useful occupation, I suspect, however humble.
And again, with guaranteed employment, even the worst job would provide a livelihood.
There are different ideas, but, I suppose that you could see it like a training program too.
Also, there are some supposed psychological benefits to work. And there is always something to do at the local community level, so it would not be totally unproductive work.