I'll take a stab at it.
If I'm correct Col signed Brannstrom on July 2 for 900k as he was let go by his former team for not being qualified.
They brought him into camp but he didn't look good enough. So they could send him down, but that costs them 900k real cash and it takes a contract spot out of their 50.
So instead they trade for Poolman's LTIR at a cost of 2M and get off Brannstrom's contract of 900k and pick up a 4th round pick for that cost. (Net cost of 1.1M cash). And Poolman's contract doesn't count against the 50. And they might already be heading into LTIR with Landeskog or Nichushkin, so if that happens, Poolman's LTIR doesn't bother them.
Just my guess.