When running casing, one of the things you always have to keep in mind is keeping the casing string filled with mud.The reason for this is, of course, that you want avoid collapse of the casing.
But filling the casing takes time. Time better spent fulfilling your primary objectives of running the casing to TD, cementing and pressure testing it. The process of filling the casing usually takes between 1 and 1.5 minutes, depending on pipe dimension, pump rates etc. 1.5 minutes is at least a quarter of the time you spend on each connection, so any time saved on this simple task might have a huge impact on the overall operation time.