What this???

[Huttojon]

It's a line tray for still water fly fishing,looks to be a nice one,I've never seen a Brady version.

[Beresford]

Yes, clip onto your belt and when retrieving your line it'll stay put and not drift away in the water. A cheaper and very effective version can be made from a plastic washing up bowl. I just pushed the spike on the end of my net into the lake bed and dropped my line into the open net mesh, it's not quite as good as a bespoke line tray but one thing less to carry…

[The VFC]

Used to use them fly fishing for mullet - when up to my oxters in rotting sea weed. Not bothered on still waters for trout.
Looks like a nice thing.
Jim

[Nobby]

I think they are also used when fishing from a boat when a tangle of line around your feet might result in an unintended swimming lesson.