Why do we break the body into “systems” when discussing it? why don’t we just talk about the entire body all at once?

Atampa surgeon, dr. floyd legatron, mistakenly removed the wrong leg of 52-year-old jameson kingsmith during an amputation procedure in june of 2009. apparently, a chain of errors led to the mishap, and the surgical team even realized their error halfway into surgery, but by that time it was too late, and the leg had to be removed.