It's supposed to act like a brace, making the handlebar more solid and prevent flexing. I suppose this can be true especially with heavy bikes and 7/8" bars instead of 1". Many off road bikes have this type of bar connecting the left and right handlebar sides already welded, so there is some science behind it. Similar to frame braces.
Also, I can attach a phone mount to it or GPS if I have it one day. At the moment maybe just a case for sunglasses or reading glasses. This bar I mounted I got from China and it is very well made despite being cheap.
1982 Kawasaki KZ1300 A4
1981 Kawasaki KZ1300 A3
assembling engine, chassis & electrical from 79 - 89 parts