If there is no in-house storage to match, how does it help the grid? It is still needed for cold winter nights, where demand is high and solar panels produce nothing. Hydro can provide the power, but the grid will be running at full load.

Depends on your system constraints.

As an example:

I live in New England. We do not have enough natural gas pipeline capacity to meet demand in long periods of very cold weather, and have very limited natural gas storage that can't buffer that for as long as a cold spell can last.

In these periods of time the grid traditionally keeps the lights on by switching over a significant portion of the grid to burning oil for power, and/or with the occasional LNG tanker load into Everett MA. These are both....pretty terrible and expensive solutions.

Burning less natural gas during the day still helps at night/at peak, because it means there's been less draw-down of our limited storage/more refill of it during the day, so we don't have to turn to worse options as heavily at night.

Most houses in Canada are heated with natural gas. I'm not negating your overall comment, but in general, cold nights don't strain the grid because of heating needs.

cold winters aren't as bad for the grid as you might expect because the cold keeps the power lines cold which lets you pump more power through them.

in-house storage helps, but net-metering and grid-storage also works