A computer is probably a good idea especially if you are just starting out. I will help you keep track of your progress in a much easier manner and give you a feel of where your fitness is.

With that said; it really all depends what kind of riding you do and what your goals are.

If you are just riding with no concern for training then you could probably do without.
If you are wanting to lose weight it might be a good idea to go with a heart rate monitor and computer to keep track of your calories and intensity of rides.

If you end up getting more serious about your training and start racing; then a power meter can be an extremely valuable tool. If you know how to use it.