These cross sections provide some really critical information about engine placement, pushrod and fuel tube routing, nose gear steering set up, and more. For example, notice in Illustration 4 that the cross hairs drawn on the firewall are off center. These marks provide the proper crankshaft placement out the front of the cowl after compensating for engine thrust offset built into the model. If you're not using the supplied engine mount, this information tells you where to center your own mount on the model to get proper engine placement. Graining is enormously helpful information I never really saw before.
Lets look at the rib cross section again.
The curved grain of the spar doubler represents basswood. The straight lines of the servo tray represent plywood, again looking into the grain, not at the flat side. This information can be enormously helpful when you're trying to identify pieces, understand a confusing section of a manual, and is especially when making repairs to a damaged model. Another place graining clarifies steps is turtle and front deck sheeting. For example, this design requires the specific strength of the sheeting as support for the fin.