If looking at your watch is enough for you, then a computer isn't needed. 🙂
I would suggest doing a search, as the pros and cons of computers are discussed in depth elsewhere...
Synopsis...
Some people love them, some people hate them, some people don't care.
What is right for you really depends on you.
Most computers include current speed, average speed, time and distance (usually an odometer and one or more trip odometers) This would be the basic.
I have read no consistent arguments concerning durability of one brand over another (but some people do have preferences).
Some have wires, some are wireless. The benefit of wireless is obvious (no wire).
Some include cadence sensors. While cadence is important to cycling, cadence can be easily computed by counting pedal strokes for 10 seconds and multiplying by 6. I personally don't use one on my computer, but would like one because I like gadgets.
Some include heart rate monitors, altimeters and other niceties.
Some mount on the handle bar, some you wear on your wrist...
Each new feature tends to add to the price.
Some want even more information, so they use hand held GPS systems like the Garmin E-Trex (the Vista looks good to me) because of the added functionality. These are mounted to bikes.
I even saw something in a forum (perhaps this one?) where a guy had 2 GPS systems on his bike, along with an air speed indicator, so he would know when he was riding into a head wind...
Others insist that the only way to train is with a PowerTap (see the Power Training forum for lots of information).
The important thing is to ride your bike.
You put on it whatever combination of gadgets you want (or leave them off if you don't want them), just ride. 😉
I have low end computers on both of my bikes because I like knowing how far I have ridden, and how fast I am going.