If you need a grid that has, say, three 9s of reliability (US grid) or four 9s reliability (European grid), then it can only happen for like 15 minutes per year

If you need a grid that has, say, three 9s of reliability (US grid) or four 9s reliability (European grid), then it can only happen for like 15 minutes per year in Europe or 150 min per year in the US. Now if you are only using wind for 10% of your power needs, a 10% drop in wind is not a big deal. But when you are using wind for 30% of your power needs, then the same variability becomes a bigger deal and might start to impact reliability, even though the wind itself is not more variable than before. Same for solar.