A concrete example is that because we don't have good data before the pandemic, it's hard to ask and answer questions whether or not the mental health of children has changed due to or after the pandemic.
If we had long-term data collection in place, we could simply ask the questions of whether or not depression and anxiety increased and we could measure it precisely with data, if we had baseline data to compare it to.
We don't have those data structures and infrastructures in place for children in mental health, and with that we don't have a lot of measures of other things that are important that are related to mental health, such as poverty, racism, violence. All of that was also increased due to the pandemic, and the pandemic influenced different people in different ways.
If we had data in place before the pandemic, then we could easily compare to see if things decreased. Without the before data, we can just say what it is now. Can we say that it changed? We can say that this is what it is now, if we collect the data now, but we don't have the ability to compare differences.