They don't seem to do anything. Searching without the brackets seems to give the same results for me.

https://www.google.com/search?q=inurl%3Abcode%3D*%2Bn_m%3D*+...

They indicate doing a search. If I say do a search for [flowers] then someone should type the actual word flowers into Google. We use the brackets to make it clear what the literal text of a search query is.