They keep it moist so the microbes and beneficial bacteria are still active when you get it and use it.

However you do want to still let the soil dry out between waterings. Not to the point the plants wilt. But enough to allow the roots to get oxygen and keep seeking out moisture. Healthy roots need oxygen to thrive and in soil the best way to get them that oxygen is through a dry cycle.

I also like to loosen up my soil periodically with a thin long metal tent stake. Usually right after watering. It helps the soil dry out some after the compacting nature of watering them. And helps oxygen penetrate past the surface.