This is used in levelling. 100 (actually 100 metres) is always added to the height above admiralty chart datum level (mean sea level) to make it easier for writing and reading. Imaging, if the height is only half a metre above mean sea level, the reading would have been 0.500 and not 100.500. For marine works, for example, heights of piles or other structures could be below sea level and if not for the adoption of adding 100, 2.5 metres below sea level would have given a reading of -2.500 instead of 97.500. Negative readings make things difficult.