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When data are summarised into classes, their exact values are lost. There is no way of knowing whether the original observations were near the bottom of the class, the top of the class or evenly spread out. Assuming that all the observations that fall into a particular class take the midpoint value is a reasonable approximation. It is the maximum likelihood unbiased estimate. It also sets the variance within each class to 0.
it gives you the midpoint of the line segment you use the formula for
A midpoint of anything is the point exactly halfway between the beginning point and the end point. So logically, it is the "midpoint".
418 to the nearest tenths = 418.0
It is its centre or the midpoint of its diameter.