Some cell lines grow as a mixed population (e.g. B95-8 - marmoset) where a proportion of cells do not attach to the tissue culture flask and remain in suspension. Therefore, to maintain this heterogeneity both the attached cells and the cells in suspension must be subcultured.
Figure 1. Examples of a semi-adherent cell line.B95-8 cells 24 hours (A) and 72 hours (B) post freeze/thaw show semi-adherent fibroblastic and lymphoblastic morphology. Confluence should carefully be monitored and passaged when cells reach ~80% confluency.