They just give a longer bolt that can take the bending stress better, those washers cone type or not don't slide around or anything, if they did they would wear out and loosen up.
The cone washers do something or they'd never have bothered using them in the past. It's not a bending stress it's more a sort of tensile stress on the bolts from the manifold heating up.
Either way something isn't right with the design. I forget what make or engine it is but for one make their solution was putting in bigger softer bolts because while they were easier to break they were more forgiving to the pressures going on in the application they were being used for.
