Daft question: Shimano brake adapter extra bit

Can anyone tell me what the extra ‘hooky type thing’ is in a shimano SM-MA-F 180 P/P2 180mm brake adaptor?

Circled in red below:

Under what circunstances do I need to fit it, and does it take the place of the domed washers?

I can’t find anything in the Shimano docs.

Stops the bolt undoing. Never really felt the need for it but I usually fit it (although I lose one every other fettle)

Install as normal and, if you want to use it, just snap it over the bolt.

Ahhh!

Right, thanks! 🙂