One tool maps the book, the other writes it.
Plottr is built for the part before and around the prose: laying scenes on a timeline, arranging threads, working from a structure template. Planners love it, and it exports the outline to the tool they draft in. By design, you do not write the manuscript in Plottr.
DraftProse is the drafting tool. The binder holds your scenes, the editor keeps you writing, and the Reader analyses the manuscript as it grows. If you are a heavy outliner, Plottr and DraftProse pair naturally: plan the shape in Plottr, write and analyse the prose in DraftProse.