Pup Joints are shorter versions of tubing and casing, used to adjust the length of a string to it´s exact requirements. There are many different cases where Pup Joints are necessary:
To reach an exact reservoir depth.
For setting packers at an exact location within the borehole.
To space out your string prior to hanging it.
Pup Joints are a mandatory part of equipment in almost any well. Due to their shorter length, they are typically easier to handle and to use. At the same time, they provide the same performance as the tubing/casing/drill pipe that they are attached to.