A hand pointing to the word pre-wash printed on top of a dishwasher detergent dispenser lid
https://media.mas.to/masto-public/media_attachments/files/111/647/234/124/724/089/original/cb997b5a40261fe9.jpg
If it didn't have a way to hold detergent back and release it later, it would all be gone after the first fill and drain (which is known as the pre-wash).
Now, because the people who made dishwashers are smart folks, they know that ideally there should be at least some detergent in that pre-wash. That's when the plates/cookware is at its very dirtiest, after all, and adding a little soap to break down those oils would help.
That's why this dispenser has a divot in the lid. It's for that.
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