Those are meant to (sort of...) convert MTB "straight" bars into some semblance of roadster bars. The reason being that generally road "drop" bars are of different diameter than MTB bars and so swapping them out (along with different brakes or brake/shifter units and all) is rather difficult.
These give you a "kind of" road bike position....
As well, I have seen people mount those old, huge regular bar-ends almost vertically on their bars to give an upright riding position. And one which causes much the same problem with brake access.