I'm confused. The filter simply restricts what parts of the JSON are returned. This can save significant bandwidth in some cases (eg listchannels but you just want the features).
Discussion
got it, i just was confused when i saw it for the first time in the commando docs the other day since it wasn’t there when i first implemented lnsocket 3 years ago…
Yes, I added it because without explicit support as a cmdline option you would get client-side filtering, which saves nothing, really.