Many cultures and religions have customs which deal with the way they use the land. for example, in judaism, it is required that a field be left fallow (unplanted) every seventh year. what is one beneficial effect of customs like this?

Field will get time to replenish its nutrient content

Explanation:

Continuous farming on any field i.e growing crops one after the other at the end of any crop season do not give  sufficient time to the field to replenish its lost nutrients due to which the next crop yield reduces.

With time the field's productivity reduces tremendously. Rituals like Judaism as mentioned in the question allows land to replenish naturally and also prepare it for next crop.

Hence, such customs are beneficial