Lets look at the oblique line being used in defense
Archers and RBT.
Archers1, RBT1 and Spear Elves2 are denied from opponent. Any attempt to reach them will take a few turns and this will maximise shooting efficiency. Spear Elves 2 will provide the final defense if the opponent reaches the shooting line.
The oblique line can prove vital in both offense and defense. It can be observed that combined arms armies are usually the ones that can make full use of both defensive and offensive properties of an oblique line. This is not to say that an army like Chaos has little use for it. In fact small armies can take full advantage of an oblique line, concentrating offensive strength on one flank of the opponent, while keeping the units near each other and not spreading too thinly over the deployment zone.
Needless to say, elite units or units led by heroes are best used in the forward wing of the oblique line, providing the "punch" needed to break the opponent's flank.