In my rather long experience with schematic capture programs, the symbols they supply are often not what you want to illustrate your idea. For example: many programs have a multiresistor pack shown as one symbol. You may want individual resistor symbols with the correct pin numbers on each. You gotta make 'em. Same goes for micros. I've never found a micro with the pins in the right places for a given schematic, so I make my own (possibly multiple versions). One classic case is the 74X240 which most capture programs show as two 4-gate units when you actually want individual gates.