Why do we use the standard uncertainty in the mean to quantify the uncertainty in the mean of a data set

To get the true value

Step-by-step explanation:

When measurements are made or calculated, some quantity from their data, they generally assume that some exact or "true value" exists based on how they define what is being measured (or calculated). Scientists reporting their results usually specify a range of values that they expect this "true value" to fall within. The most common way to show the range of values is:

measurement = best estimate ± uncertainty