I think the angle feature is if you're building a multi sided shape like a hexagon, say for a table. I got the whole kit for my birthday back in August but haven't found a use for it as of yet. Having the MTR-18 triangle with the newly introduced taller threaded pegs that register off your guide rail for squaring the rail to the fence is pretty awesome. The angle brackets for clamping inside a carcass for squaring it up are nice. The angle adjust I'm sure I'll use some day but haven't seen the need yet.
TSO keeps making improvements/adjustments to the accessory kit and they aren't the best at describing how everything works. Mike's Woodshop on YouTube did a nice video on the MTR-18 that helped me understand how it all goes together. TSO seems to be investing into their own YouTube channel and hopefully will put together a real life usage video of the triangle as I think it has a lot of potential.