Thanks! It's been a long time since I tried APL. All I could remember was ⍳ and right-to-left evaluation. The rest was through monkeying around with Rosetta Code examples.

As to your "why" question - I'm assuming to make a neat demo. Something like:

  >>> from math import cos, sin
  >>> [s for i in range(50) if i&1 and cos(s:=i*i) < sin(i)]
  [1, 9, 49, 225, 361, 441, 625, 1089, 1521, 1681, 2025, 2209]

seems more difficult to pull off in a pipeline API.