That's so that the tweaks that are made spread to the entire organism, rather than affecting only a small group of cells. (businessinsider.com)
Scientists made select nerve cells in mice's brains sensitive to light, and then used lasers to activate specific groups of those cells. (sciencenews.org)
When groups of cells work together to perform a specific function, they form a tissue. (sciencenewsforstudents.org)