So they're lending money to the government, who spends it.

All BigCorp Ireland LTD has is a brokerage account full of ISIN codes.

That doesn’t sound like a typical business argument. We could more efficiently get the money in the government treasury by skipping the corporate middleman and taxing it.

How are those tbills delivering value to the shareholders?

Collecting and accounting for tax is far far more inefficient than buying electronic tbills.

And they pay interest to the holder...

The point is, the money isn't keeping out of the economy. It's loaned to the government who spends it.