The idea is simply that rebuilding the tips commonly calls for making new decks, splicing in new wood to the existing inwales, and splicing in a new section for the stem, or replacing the stem entirely. The challenge is that all these four pieces must be fit together very precisely and then fastened together very strongly. When building a canoe from scratch on a form, it’s relatively easy to make stem, inwales and decks come together right, as the form holds everything in place. When doing restoration, it’s another matter.
So this sub-deck is sort of a “mini-form” that lets you custom fit all these pieces into a new tight joint. Plus it lets you to secure the top of the stem into something other than the inwales—which don’t have a lot of room for a mortise anyway.