What is the function of introducing cement plug before pouring concrete in bored piles?

It is a common practice in the construction of bored piles by adding a cement plug before concreting of bored piles takes place. One of the possible explanations is that the cement grout serves as a barrier and protects fresh concrete from being washed away by water inside pile bore. The cement grout helps to set aside water when the first batch of concrete is poured down from the tremie pipe. As such, the quality of fresh concrete is anticipated not to be seriously affected by water and this helps to improve toe imperfection problem.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.