If I need (for my small European country) 15 TWh of electricity per year, and my typical rainfall onto catchment area for hydro is enough to do maybe 10 TWh per year, I'm 5 TWh short. In the 1980s you'd have probably solved that with CCGT, but that means finding or importing methane (natural gas). If I also build 10 TWh of wind production, even though I can't tell you a year in advance which days it will blow hardest, it will blow, and now my total capacity is enough to run solid green, because when the wind blows the hydro schemes save water to use another day.

Wind production varies year on year, but only maybe 10% where the variation day to day is almost 100%. So wind + hydro should pair nicely because the hydro plant can ramp up or down production as appropriate.