Transcription factors regulate the genes in different cells. That is, different cells have different sequence of genes that are transcribed (set on or off) by the RNA polymerase. This will help different types of cells to perform their own unique operation.
So, why do the cells have to look different physically, if the their function is determined by the on/off sequence?
NOTE: I am a neophyte in Bioinformatics. Any help on my building my conceptual knowledge is appreciated.