What should one think about this?

It depends on how badly you want/need this particular job, how much you trust this employer and what are the norms in your locale and domain. Consider your alternatives before making this decision.

I can't imagine doing this in my part of the world and my domain (US, software). If someone wants to hire me, they would need to pay for all of my work.

But working a few month is not typical in my space, and I'm not desperate for a job. Your mileage may vary.

What should one think about this?

It depends on how badly you want/need this particular job, how much you trust this employer and what are the norms in your locale and domain. Consider your alternatives before making this decision.

I can't imagine doing this in my part of the world and my domain (US, software). If someone wants to hire me, they would need to pay for all of my work.

But working a free month is not typical in my space, and I'm not desperate for a job. Your mileage may vary.