The first layer of trim is the 3 inch boards that flank the lapstrake siding. These boards will bring the cosmetic layer flush to the top of the siding, allowing the 5 inch boards to cover the siding without any obstructions.
The 3 inch boards are simply fitted as close as possible to the siding's edge with little regard for perfection. The gaps will be filled with caulk and this mess will be covered by the 5 inch trim board. The 3 inch boards simply bring the 5 inch board up to the level of the siding so that there is no interference and the trim boards can go on effortlessly and cleanly.
The trim that lines the roof is very important because it tacks the tar paper in place wrapping over the edge of the roof. The shingles will cover this area and provide extra water protection.
Battens were added on the roof to hold the tar paper in place and avoid any tears. If the tar paper were left free over the roof, it could rip off in strong winds and negate any of the benefits of tacking the tar paper under the trim.