To help yourself is to take something without asking. This can be with or without the owner's permission: There was plenty of food at the Maria's party. She told us to help ourselves. (meaning she didn't serve them. She gave them permission to serve themselves) The store was closed. The thieves helped themselves to all the goods. (they robbed the store).

We helped ourselves to the food means -- we took the food ourselves.

We helped ourselves to the food*