What are they for? I'm trying to imagine a situation where I'd want to hold
two pieces of tubing in parallel and coming up blank. (Admittedly, I'm not
all that bright.)

When the bimini is removed from the boat ( I always remove it when
trailering,) the 7/8" diam support poles swing freely and can be
snapped on to the main support poles to secure them.
You would have to see it to see exactly, but it's just a matter of
clipping two 7/8" poles together for easy transportation.
I could use bungees or something, but these pole clips have been very
convenient ( but easy to lose.)
John C.