Why is cholera toxin added to certain culture media?
Empirically, it can improve growth of cell cultures (https://www.atcc.org/Global/FAQs/C/C/Cholera%20toxin-541.aspx).

It furthermore has the potential to make cell populations more homogeneous by selectively binding to gm1 containing cells that can arise from spatial inhomogeneities among cultured cells. Snijder er al. Nature 2009 (https://www.ncbi.nlm.nih.gov/pubmed/19710653)