Stable expression in a cell lines is very useful for numerous projects. For example, it helps in Eukaryotes for optimization of protein productions and it is convenient for recurrent needs and high yields. Stable exogenous expression of tagged protein allows extensive tracking and localisation of the protein in live. Cell lines stably expressing a protein can be used for various screenings such as lost of function assays (CAS9-expressing cells), and reporter-based assays (Gaussian luciferase). Of course, the stable expression could also be shRNA for knock-down.
Stable expression implies insertion into the genome. It used to be random and could cause hits with side effects. Now, targeted insertion in Safe-Harbor sites is now possible. How can you take advantage of this progress?