Tissue cultures (samples of cancer cells) are taken from cancer patients, and the cells are then grown in a laboratory. What is the purpose of growing cancer cells in a laboratory?

For observation and research. This is to analyse their features, structures, physiology and other possible mutations - genetic structures that have caused it to arrive at this rate.

Cancer cells are the result of those cells that deviate from the normal life cycle of a cell. These cancer cells refuse to die and rapidly increase in number due to the uncontrollable burst of cell division.

The anwser is to test medicinal treatments before giving them to patients.