Mon Sotah 14

The Gemara says the reason that the Sotah carries around her Mincha is to tire her out so she shouldn't resist admitting.

Tosfos points out that this fits also to those that don't hold that we Darshin the Torah's reason. That argument is only when the Torah says a Halacha which infers that it implied by all cases, and by Darshening the reason we limit the places to apply it. Like the prohibition of taking a collateral from a widow. The reason R' Shimon says is since you need to return it every day, your meetings will cause suspicion. Therefore if she's rich and you don't need to return the collateral, then there is no prohibition, although the Toarh doesn't differentiate and seems to reefer to all widows.

However, here where we don't limit the cases the Halacha applies to, we could give some reason for it.