One of the major steps in human carcinogenesis is cellular immortalization, a process by which the cells must escape senescence and can acquire an infinite lifespan.
In the absence of immortalization, however, a cell might undergo malignant transformations that could not proliferate indefinitely.
In cell culture, primary cells enter in a replicative senescence state, when morphology, gene expression, and metabolism gets altered.
Therefore, scientists started immortalizing primary cells in vitro to study the cell growth, differentiation, and senescence using continuous cell lines.
Cancer cells have been said to be immortal because, unlike normal cells, they stop aging and die, instead continue to multiply endlessly.