The dried leaves makes a nice tea (there was reportedly a company in Norway that imported 1-2 tons of ceylon tea, and sold 15 tons, the remainder was rosebay willowherb. I use it with a little bit of mint and backcurrent leaves mixed in; quite nice, tastes a bit like a "herby" green tea.
The pith is edible and tasts nice.
The roots are avlos edible, but not so nice IIRC. Can't recall the details, atm, but recall there was only some portion of the roots that was usefull. Getting a bit late in season for them too, at least around here.
Fluff for fire-starting, of course.