I'm just going by my experince when using FO's and i don't use them them much.
When i have, they seem have a slower call than the the AO and more obs eyes on targets,when in contact,make a difference on AO artry calls,esp in WW2 (which i play mainly) ,in MBT that may be different
Another point with obs, is you can place "gold spots" and have low call for arty on the spots.