What is the significance of reinforced concrete infill in marine piling system of steel tubular pile with reinforced concrete infill?

Reinforced concrete is designed to fill the void space inside the steel tubular piles from pile cap to a certain distance below seabed. As mentioned earlier, steel tubular piles above seabed level is assumed in design to be completely corroded when approaching the end of design life. As such, loads from pile caps are transferred directly to reinforced concrete infill instead of steel tubular piles. The load transfer path below seabed level is as follows: loads from reinforced concrete infill are transferred to steel tubular piles through frictional force between reinforced concrete infill and steel piles. Therefore, mobilization of frictional forces between reinforced concrete infill and steel piles is essential to ensure that the piling system functions properly.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.