The idea is to avoid using pumps. The goal is to treat the grapes and wine gently. This is especially true prior to the wine going in to barrels. The term “crush” is misleading since in making fine red wines, you don’t want to crush the grapes but rather just break open the berries allowing the juice to escape without breaking the seeds. Broken seeds can impart a harsh flavor to the wine.
Gravity flow wineries are designed so that you never (or rarely) need to pump the wine anywhere. You remove the possibility that some seeds will be crushed by a pump. It is also gentler on the finished wine. This, in theory, leads to a better wine.