The first international standard of length was a metal bar kept at the International Bureau of Weights and Measures. One meter of length was defined to be the distance between two fine lines engraved near the ends of the bar. Why was it important that the bar be kept at a constant temperature?

International Bureau of Weights and Measures have the standard bar for one meter length and it is made up of the platinium-iridium alloy. As these are metals so they have the thermal expansion. So the length of the bar depends upon the temperature. Temperature changes can effect the length of the standard bar. That's the reason behind keeping the bar at the constant temperature as the change in temperature will result in the change of length.