I'm pretty sure that's more or less the same structure used by WinCo as opposed to direct ownership

Sure, my understanding was that Galois is not exactly a co-op.

Why this structure? Is it more advantageous in the US?

One of the main challenges for co-ops is navigating the entry and exit. If you had Direct employee ownership, you have to buy in when you start and sell your stock back when you leave.

A trust with the employees as beneficiaries solves a lot of these problems. It also prevents the co-op members from taking the company public for a huge payday.