I am primarily a utility cyclist. I use my bike to go places. I get on. I pedal until I get there, and then I stop. I repeat this several times a day. I don't think a computer will help me with this much, although I sometimes get curious about how far or fast I go, I have not been motivated to spend money on a computer.
If I were a fitness cyclist, I don't see how I could get along without one. Intervals would be difficult, and it is always better to measure than estimate if you can.
Dan