The thin walled steel tube is known as the casing, and it's purpose is build up a secure mould for casting concrete that does not contain any earth and debris. Once the casting is finished, it does carry out any structural role.

Some soils are extremely cohesive. It signifies that when a hole is bored into the soil like 1 foot wide by 50 feet deep, then the soil retains the shape of the hole and does not fall into the hole and block it. If such soil exists at the site, then it is not required to provide a casing in place: the casing can be applied to bore the hole for the pile, and then eliminate it, and then cast the pile in place. In this way, huge cost is saved since the similar casing tube is utilized to bore holes for all the piles.