by ad_hoc
The idea is that you can sow multiple fields with the same sow action. Also the grain and veggies are worth points at the end of the game even if they are still in your fields.For example, if you have 2 grain and 1 veggie in your supply (and 3 fields) with 1 action you can sow them to have 6 grain and 2 veggies which is quite economical (even better if you do it with the plow/sow action).
Typically it is not very good to sow early in the game. Instead you should be focusing on building a 3rd room so you can get another family member and acquiring food like fishing or cooking sheep.